UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 10-K

Aon Corporation
(Exact Name of Registrant as Specified in Its Charter)

        Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. YES ý    NO o

        Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant's knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    o

        Indicate by check mark whether the registrant is an accelerated filer (as defined in Exchange Act Rule 12b-2). YES ý    NO o

        Aggregate market value of common stock held by non-affiliates of the registrant as of June 30, 2004 was $8,297,300,700.

        Number of shares of common stock outstanding as of February 28, 2005 was 317,464,955.

Documents incorporated by reference:

        Portions of Aon Corporation's Proxy Statement for the 2005 Annual Meeting of Stockholders to be held on May 20, 2005 are incorporated by reference in this Form 10-K in response to Part III, Items 10, 11, 12, 13 and 14.

PART I

Item 1.    Business

Overview

        Aon Corporation ("Aon"), through its various subsidiaries worldwide, serves its clients through three operating segments:

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Risk and Insurance Brokerage Services acts as an advisor and insurance broker, helping clients manage their risks, and negotiating and placing insurance risk with insurance carriers through our global distribution network.



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Consulting provides advice and services to clients for employee benefits, compensation, management consulting, communications, human resource outsourcing and strategic human resource consulting.



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Insurance Underwriting provides specialty insurance products, including supplemental accident, health and life insurance; credit life, accident and health insurance; extended warranty products, and select property and casualty insurance products and services.

        Our clients include corporations and businesses, insurance companies, professional organizations, independent agents and brokers, governments, and other entities. We also serve individuals through personal lines, affinity groups, and certain specialty operations.

        Incorporated in 1979, Aon is the parent corporation of long-established and more recently formed companies. Aon has approximately 48,000 employees and does business in more than 120 countries and sovereignties.

Segment Operations

Risk and Insurance Brokerage Services

        The Risk and Insurance Brokerage Services segment generated approximately 57% of our total operating segment revenues in 2004. This is the largest of our operating segments, with approximately 31,000 employees worldwide. Risk and insurance brokerage services are provided by certain indirect subsidiaries, including Aon Risk Services Companies, Inc.; Aon Holdings International bv; Aon Re Worldwide, Inc.; Aon Limited (U.K.); and Cananwill, Inc.

Subsegments

        We measure our revenues in this segment under the following areas:

        Risk Management and Insurance Brokerage encompasses our retail and wholesale brokerage services, affinity products, managing general underwriting, placement and captive management services, and premium finance services for small, mid-sized and large companies, including Fortune 500 corporations. The Americas' operations provide products and services to clients in North and South America, the Caribbean and Bermuda. Our International operations offer similar products and services to the rest of the world. Risk management services also include risk identification and assessment, safety engineering, claims and loss cost management, and program administration.

        Retail brokerage has practice areas to deliver specialized advice and services in such segments as entertainment, media, financial institutions, marine, aviation, construction, healthcare and energy, among others.

        As a retail broker, Aon generally serves as an advisor to corporate clients and can arrange a wide spectrum of risk management solutions, including property, general liability, professional and directors' and officers' liability, workers' compensation, and other exposures. Aon also provides affinity products

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for professional liability, life, disability income and personal lines for individuals, associations and businesses.

        Wholesale brokerage operations serve retail insurance brokers and independent agents in placing large and small accounts with both standard and specialty carriers. Our wholesale brokerage operations are a commercial resource for insurance products, specialty programs and exclusive underwriting facilities.

        In our managing general underwriting business, we provide outsourced solutions to insurance companies, such as risk selection, premium rating, form design and client service.

        Aon's wholesale brokers and managing general underwriting units offer more than 450 insurance products and programs. Clients may access them directly, or through the Aon Specialty Product Network (ASPN), which we developed as a single-point-of-contact for agent and broker clients who need specialty insurance solutions for their customers.

        We are also a major provider in the management of captive insurance companies that enable our clients to manage risks that would be cost prohibitive or unavailable in traditional insurance markets.

        Reinsurance Brokerage and Related Services offers sophisticated advisory services in program design and claim recoveries that enhance the risk/return characteristics of insurance policy portfolios, improve capital utilization and evaluate and mitigate catastrophic loss exposures worldwide. An insurance or reinsurance company may seek reinsurance or other risk-transfer financing on all or a portion of the risks it insures. Brokerage services use dynamic financial analysis and capital market alternatives, such as transferring catastrophe risk through securitization.

        Aon Re Worldwide, Inc., its subsidiaries and its affiliates provide reinsurance services to insurance and reinsurance companies and other risk assumption entities by acting as brokers or intermediaries on all classes of reinsurance. While property and casualty lines dominate, our reinsurance activities also include specialty lines such as professional liability, medical malpractice, accident, life and health. Services include advice, placement of reinsurance and alternative risk transfer financing with capital markets, and related services such as actuarial, financial and regulatory consulting, portfolio analysis, catastrophe modeling, and claims services.

        Claim Services offered claims administration and loss cost management services through dedicated subsidiaries that are separate from our risk management and reinsurance brokerage services. In the United States, these services were delivered principally through Cambridge Integrated Services Group ("Cambridge"). During 2004, we exited most of these activities by completing the sale of our U.K. claims operations in the second quarter 2004 and our Cambridge business in fourth quarter 2004.

Compensation for Services

        Revenues are generated through commissions, fees from clients, and compensation from insurance and reinsurance companies with whom we place business for services provided to them. On October 22, 2004, we announced that we are terminating our contingent commission arrangements with underwriters. We have nearly completed this process and we are working closely with our clients, insurance carriers, regulators and others to establish a new business model that is transparent, easy to understand and accepted by clients.

        Commission rates and fees vary, depending upon several factors which may include the amount of premium, the type of insurance or reinsurance coverage provided, the particular services provided to an insurer or reinsurer, and the capacity in which the broker acts. We also receive investment income on funds held on behalf of clients.

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Competitive Conditions

        We believe we are the second largest insurance broker worldwide based on total revenues. The risk and insurance brokerage services business is highly competitive and we compete with two other global brokers in addition to numerous specialist, regional and local firms in almost every area of our business; insurance and reinsurance companies that market and service their insurance products without the assistance of brokers or agents; and with other businesses, including commercial and investment banks, accounting firms and consultants that provide risk-related services and products.

Consulting

        The Consulting segment generated approximately 12% of our total operating segment revenues in 2004. It has approximately 7,000 employees worldwide located in more than 120 offices, with operations in the United States, Canada, Europe, Asia/Pacific and South Africa. We believe we are the world's third largest employee benefit consultant and the second largest in the United States based on total revenues.

Subsegments

        Through our Aon Consulting Worldwide, Inc. subsidiary (Aon Consulting), we provide a full range of human capital consulting services in two subsegments (Consulting Services and Outsourcing) that operate in six practice areas:

        Employee Benefits advises clients regarding the structure, funding and administration of employee benefit programs, which attract, retain and motivate employees. Benefits consulting includes health and welfare, retirement, executive benefits, absence management, compliance, employee commitment, investment advisory and elective benefits services.

        Human Resource Outsourcing offers employment processing, performance improvement, benefits administration, and other employment-related services.

        Compensation focuses on designing salary, bonus, commission, stock option, and other pay structures, with special expertise in the financial services and technology industries.

        Management Consulting assists clients in process improvement and design; leadership, organization and human capital development; and change management.

        Communications advises clients on how to communicate initiatives that support their corporate vision.

        Strategic Human Resource Consulting advises complex global organizations on talent, change and organization effectiveness issues including assessment, selection performance management, succession planning, organization design and related people-management programs.

        Aon Consulting works to maximize the value of clients' human resources spending, increase employee productivity, and improve employee performance. Its approach addresses a trend toward more diverse workforces (demographics, nationalities, cultures and work/lifestyle preferences) that require more choices and flexibility among employers—with benefit options suited to individual needs.

        Our consulting professionals and their clients also identify options in human resource outsourcing and process improvement. Prime areas where companies choose to use outsourcing services include the assessment and selection of job candidates, employment processing, training and development, benefits administration and the individual benefits enrollment process.

Compensation for Services

        Aon Consulting revenues are principally derived from fees paid by clients for advice and services. In addition, commission revenue is received from insurance companies for the placement of individual

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and group insurance contracts, primarily life, health and accident coverages. On October 22, 2004, we announced that we are terminating our contingent commission arrangements with underwriters. We have nearly completed this process and we are working to establish a new business model that is transparent, easy to understand, and accepted by clients.

Competitive Conditions

        Our consulting business faces strong competition from other privately and publicly held worldwide and national consulting companies, as well as regional and local firms. Competitors include independent consulting firms and consulting organizations affiliated with accounting, information systems, technology and financial services firms. Some of our competitors provide administrative or consulting services as an adjunct to other primary services.

Insurance Underwriting

        Our insurance underwriting segment, with approximately 9,000 employees worldwide, has operations in the United States, Canada, Latin America, Europe and Asia/Pacific. This segment generated approximately 31% of Aon's total operating segment revenues in 2004.

Subsegments

        We classify our insurance underwriting businesses into two sub-segments: (1) accident & health and life, and (2) warranty, credit and property and casualty.

Accident & Health and Life

        Our Combined Insurance Company of America and Combined Life Insurance Company of New York (Combined) subsidiaries provide supplemental accident, health and life insurance. We are a leading underwriter and distributor of specialty individual accident, disability, health and life insurance products that are targeted to middle income consumers in the United States, Europe, Canada, and Asia/Pacific.

        A worldwide sales force of approximately 6,800 exclusive career agents service clients regularly to initiate and renew coverage and to sell additional coverage. We offer a wide range of accident and sickness insurance products, including short-term disability, critical conditions and cancer aid, Medicare products, hospital confinement/recovery, and long-term care coverage. Most of these products are primarily fixed-indemnity obligations, and are not subject to escalating medical cost inflation.

        With the continuing rise of employee benefit costs, Combined also works with employers to contain those costs while offering quality benefits that appeal to their employees' individual needs. For example, a Worksite Solutions program complements existing benefits packages offered by employers with no additional cost to a company. Individual employees choose among supplemental insurance product options and pay for them through payroll deductions.

Compensation for Services

        Accident and health revenues are based on premiums paid by policyholders for insurance coverage and services.

Competitive Conditions

        The accident and health insurance industry in the United States is highly diverse, with more than 1,500 accident and health and life insurance companies competing in various segments of the industry. We believe that competition in our accident, health and life business is based on service, product features, price, commission structure, financial strength, claims-paying ability ratings and name recognition.

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Warranty, Credit and Property and Casualty

        We believe we are the world's largest independent provider of extended warranty products. These products are offered through our Virginia Surety Company, Inc. and London General Insurance Company Limited subsidiaries.

        Extended warranty, which is the largest line of business in this sub-segment, offers extended service plans and warranties for:

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some of the world's premier manufacturers, distributors and retailers of many types of consumer goods including automobiles, electronics, appliances, computers and telephone equipment



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home buyers and sellers.

        Other products include extended warranty or insurance protection for items purchased with a credit card and extended warranties on major home systems and appliances.

        Services include compliance support, merchandising, direct marketing, training, and customer care management services. Products are sold through retailers, automotive dealers, insurance agents and brokers, and real estate brokers.

        Our credit life, accident and health, and disability insurance provides coverage for unpaid loans in the event of death, illness, accident or involuntary unemployment. This insurance is sold by automobile dealers with automobile financing, and by financial institutions with consumer loans.

        Select property and casualty products are designed to protect businesses against losses related to various personal and commercial risks, such as professional liability errors and omissions, excess liability, and workers' compensation. We offer select commercial property and casualty business on a limited basis through managing general underwriters, primarily Aon-owned companies.

Compensation for Services

        Insurance revenues are based on premiums paid by policyholders. Certain other revenues are based on fees paid by clients for administrative and other services.

Competitive Conditions

        We believe that competition in our warranty, credit and specialty property and casualty business is based on service, product features, price, commission structure, financial strength, claims-paying ability, ratings and name recognition. In our extended warranty business, we compete with a large number of insurance companies and other financial services providers in addition to third-party administrators, manufacturers and distributors.

Disposal of Operations

        The Registrant hereby incorporates by reference Note 5, "Disposal of Operations," of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

Licensing and Regulation

        Regulatory authorities in the states or countries in which the operating subsidiaries of Aon Group conduct business may require individual or company licensing to act as brokers, agents, third party administrators, managing general agents, reinsurance intermediaries or adjusters. Under the laws of most states in the U.S. and of most foreign countries, regulatory authorities have relatively broad discretion with respect to granting, renewing and revoking brokers' and agents' licenses to transact business in the state or country. The manner of operating in particular states and countries may vary according to the licensing requirements of the particular state or country, which may require, among other things, that a firm operate in the state or country through a local corporation. In a few states and

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countries, licenses are issued only to individual residents or locally owned business entities. In such cases, Aon Group subsidiaries have arrangements with residents or business entities licensed to act in the state or country.

        Insurance companies must comply with laws and regulations of the jurisdictions in which they do business. These laws and regulations are designed to ensure financial solvency of insurance companies and to require fair and adequate service and treatment for policyholders. They are enforced by the states in the U.S., by the Financial Services Authority ("FSA") in the United Kingdom, and by various regulatory agencies in other countries through the granting and revoking of licenses to do business, licensing of agents, monitoring of trade practices, policy form approval, minimum loss ratio requirements, limits on premium and commission rates, and minimum reserve and capital requirements. Compliance is monitored by the state insurance departments through periodic regulatory reporting procedures and periodic examinations. The quarterly and annual financial reports to the regulators in the U.S. utilize statutory accounting principles which are different from U.S. generally accepted accounting principles. The statutory accounting principles, in keeping with the intent to assure the protection of policyholders are based, in general, on a liquidation concept, while U.S. generally accepted accounting principles are based on a going-concern concept.

        The state insurance regulators are members of the National Association of Insurance Commissioners ("NAIC"). The NAIC seeks to promote uniformity of, and to enhance the state regulation of, insurance. Both the NAIC and the individual states continue to focus on the solvency of insurance companies and their conduct in the marketplace. This focus is reflected in additional regulatory oversight by the states and emphasis on the enactment or adoption of a series of NAIC model laws and regulations designed to promote solvency.

        The NAIC has a formula for analyzing insurers called risk-based capital ("RBC"). RBC establishes "minimum" capital threshold levels that vary with the size and mix of a company's business. It is designed to identify companies with capital levels that may require regulatory attention.

        The state insurance holding company laws require prior notice to, and approval of, the domestic state insurance department of intracorporate transfers of assets within the holding company structure, including the payment of dividends by insurance company subsidiaries. In addition, the premium finance loans by Cananwill, our indirect wholly owned subsidiary, are subject to one or more truth-in-lending and credit regulations, insurance premium finance acts, retail installment sales acts and other similar consumer protection legislation. Failure to comply with such laws or regulations can result in the temporary suspension or permanent loss of the right to engage in business in a particular jurisdiction as well as other penalties.

        Beginning in January 2005, our principal subsidiary in the U.K., Aon Limited, must be authorized by the FSA. Previously, Aon Limited was a member of a self-regulatory body. Regulation by the FSA has been introduced pursuant to the European Insurance Mediation Directive, which sets minimum standards for those involved in advising on, arranging, administering or introducing contracts of insurance. The regulation requires significant operational changes, for example, enhanced disclosures, particularly in connection with retail (private and non-commercial) customers. The FSA has also indicated that it will adopt rules regarding use of funds held on behalf of clients that will have significant consequences for all brokers operating in the London market.

Clientele

        No significant part of our or our subsidiaries' business is dependent upon a single client or on a few clients, the loss of any one of which would have a material adverse effect on us or our operating segments.

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Employees

        At December 31, 2004, our operating subsidiaries had approximately 48,000 employees, of whom approximately 44,500 are salaried and hourly employees and the remaining 3,500 are career agents who are generally compensated wholly or primarily by commission. In addition, there were approximately 3,300 international career agents who are considered independent contractors and are not our employees. Of the total number of employees, approximately 21,000 work in the U.S.

Risks Related to Our Business and the Insurance Industry

Our results may fluctuate due to many factors, including cyclical or permanent changes in the insurance and reinsurance industries.

        Our results historically have been subject to significant fluctuations arising from uncertainties and changes in the insurance industry. Changes in premium rates affect not only the potential profitability of our underwriting businesses but also generally affect the commissions and fees payable to our brokerage businesses. In addition, insurance industry developments that can significantly affect our financial performance include factors such as:

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rising levels of actual costs that are not known by companies at the time they price their products;



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volatile and unpredictable developments, including weather-related and other natural and man-made catastrophes, including acts of terrorism;



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changes in levels of capacity and demand, including reinsurance capacity;



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changes in reserves resulting from the general claims and legal environments as different types of claims arise and judicial interpretations relating to the scope of insurers' liabilities develop; and



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changes in business practices and business compensation models.

Our results may be adversely affected if we are unable to successfully implement a new business model.

        In October 2004, we announced that we were terminating contingent commission arrangements with underwriters. In connection with the elimination of contingent commissions, we are in the process of establishing a new business compensation model. There is no assurance that we will be able to develop an effective new business compensation model, nor can we assure that any new business compensation model we develop will generate revenues equivalent to those previously received from contingent commissions.

A further decline in the credit ratings of our senior debt and commercial paper may adversely affect our borrowing costs and financial flexibility.

        In 2004, Standard & Poor's (S&P) lowered its ratings on our senior debt to the current rating of "BBB+" from "A-". In addition, S&P placed all their ratings for Aon on credit watch with negative implications. Also in 2004, Moody's Investor Services (Moody's) and Fitch, Inc. placed both our senior debt and commercial paper ratings on negative outlook and credit watch with negative implications, respectively.

        In March 2005, Fitch, Inc. lowered its ratings on our senior debt from "A-" to "BBB+" and affirmed our commercial paper rating of "F2." Their rating outlook continues to be negative. S&P affirmed its ratings for Aon and removed us from credit watch. Moody's affirmed its ratings on our senior debt and changed their outlook from negative to stable. A downgrade in the credit ratings of our senior debt and commercial paper would increase our borrowing costs and reduce our financial flexibility.

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        Any such further downgrade may trigger obligations of our company to fund certain amounts with respect to our premium finance securitizations, including, in the event of a downgrade by Moody's prior to April 28, 2005, up to $61 million with respect to our U.S. facility. Moreover, some of our debt instruments, such as our 6.20% notes due January 2007 ($250 million of which are outstanding), expressly provide for interest rate increases in the case of certain ratings downgrades. Similarly, any such downgrade would increase our commercial paper interest rates or may result in our inability to access the commercial paper market altogether. We cannot assume that our financial position would not be adversely affected if we are unable to access the commercial paper market. A downgrade in the credit ratings of our senior debt may also adversely affect the claims-paying ability or financial strength ratings of our insurance company subsidiaries. See "A decline in the financial strength or claims-paying ability ratings of our insurance underwriting subsidiaries may increase policy cancellations and negatively impact new sales of insurance products" below.

We face significant competitive pressures in each of our businesses.

        We believe that competition in our lines of business is based on service, product features, price, commission structure, financial strength, claims-paying ability ratings and name recognition. In particular, we compete with a large number of national, regional and local insurance companies and other financial services providers, brokers, and with respect to our extended warranty business, third-party administrators, manufacturers and distributors.

        Some of our underwriting competitors have penetrated more markets and offer a more extensive portfolio of products and services and have more competitive pricing than we do, which can adversely affect our ability to compete for business. Some underwriters also have higher claims-paying ability ratings and greater financial resources with which to compete and are subject to less government regulation than our underwriting operations.

        We encounter strong competition for both business and professional talent in our insurance brokerage and risk management services operations from other insurance brokerage firms which also operate on a nationwide or worldwide basis, from a large number of regional and local firms throughout the world, from insurance and reinsurance companies that market and service their insurance products without the assistance of brokers or agents, and from other businesses, including commercial and investment banks, accounting firms and consultants that provide risk-related services and products. Our consulting operations compete with independent consulting firms and consulting organizations affiliated with accounting, information systems, technology and financial services firms around the world.

        In addition, the increase in competition due to new legislative or industry developments could adversely affect us. These developments include:

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an increase in capital-raising by insurance underwriting companies, which could result in new entrants to our markets and an influx of capital into the industry;



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the selling of insurance by insurance companies directly to insureds;



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changes in our business compensation model as a result of regulatory investigations;



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the establishment of programs in which state-sponsored entities provide property insurance in catastrophe prone areas or other alternative markets types of coverage; and



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additional regulations promulgated by the Financial Services Authority in the U.K.

        New competition as a result of these developments could cause the supply of, and demand for, our products and services to change, which could adversely affect our results of operations and financial condition.

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A decline in the financial strength or claims-paying ability ratings of our insurance underwriting subsidiaries may increase policy cancellations and negatively impact new sales of insurance products.

        Financial strength and claims-paying ability ratings have become increasingly important factors in establishing the competitive position of insurance companies. These ratings are based upon criteria established by the rating agencies for the purpose of rendering an opinion as to an insurance company's financial strength, operating performance, strategic position and ability to meet its obligations to policyholders. They are not evaluations directed toward the protection of investors, nor are they recommendations to buy, sell or hold specific securities. Periodically, the rating agencies evaluate our insurance underwriting subsidiaries to confirm that they continue to meet the criteria of the ratings previously assigned to them. A downgrade, or the potential for a downgrade, of these ratings could, among other things, increase the number of policy cancellations, adversely affect relationships with brokers, retailers and other distributors of our products and services, negatively impact new sales and adversely affect our ability to compete.

        Virginia Surety Company, Inc., our principal property and casualty insurance company subsidiary, is currently rated "A-" (excellent; fourth highest of 16 rating levels) by A.M. Best Company. Combined Insurance Company of America, the principal insurance subsidiary that underwrites our specialty accident and health insurance business, is currently rated "A" (excellent; third highest of 16 rating levels) by A.M. Best Company, "BBB+" (good; fourth highest of nine rating levels and highest ranking within the level) for financial strength by S&P and "A3" (good; third highest of nine rating levels) for financial strength by Moody's Investors Service. We cannot assure that one or more of the rating agencies will not downgrade or withdraw their financial strength or claims-paying ability ratings in the future.

Changes in interest rates and investment prices could reduce the value of our investment portfolio and adversely affect our financial condition or results.

        Our insurance underwriting subsidiaries own a substantial investment portfolio of fixed-maturity and equity and other long-term investments. As of December 31, 2004, our fixed-maturity investments (approximately 97% was investment grade) had a carrying value of $3.5 billion, our equity investments had a carrying value of $40 million and our other long-term investments and limited partnerships had a carrying value of $483 million. Accordingly, changes in interest rates and investment prices could reduce the value of our investment portfolio and adversely affect our financial condition or results.

        For example, changes in domestic and international interest rates directly affect our income from, and the market value of, fixed-maturity investments. Similarly, general economic conditions, stock market conditions and other factors beyond our control affect the value of our equity investments. We monitor our portfolio for other-than-temporary impairments in carrying value. For securities judged to have an other-than-temporary impairment, we recognize a realized loss through the statement of income to write down the value of those securities.

        For 2004, we recognized impairment losses of $3 million. We cannot assure that we will not have to recognize additional impairment losses in the future, which would negatively affect our financial results.

        On December 31, 2001, our two major insurance companies sold the vast majority of their limited partnership portfolio, valued at $450 million, to Private Equity Partnership Structures I, LLC, (PEPS I) a qualifying special purpose entity (QSPE). The common stock interest in PEPS I is held by a limited liability company which is owned by one of our subsidiaries (49%) and by a charitable trust, which is not controlled by us, established for victims of the September 11 attacks (51%). Approximately $171 million of investment grade fixed-maturity securities were sold by PEPS I to unaffiliated third parties. PEPS I then paid our insurance underwriting companies the $171 million in cash and issued to them an additional $279 million in fixed-maturity and preferred stock securities. The fixed-maturity

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securities our insurance underwriting companies received from PEPS I are rated as investment grade by S&P.

        As part of this transaction, our insurance underwriting companies were required to purchase from PEPS I additional fixed-maturity securities in an amount equal to the unfunded limited partnership commitments, as they are requested. Beginning in July 2004, Aon Parent Company is funding all future commitments. As of December 31, 2004, these unfunded commitments amounted to $60 million.

        Although the PEPS I transaction has reduced the reported earnings volatility historically associated with directly owning limited partnership investments, it will not eliminate our risk of future losses. For instance, we must analyze our preferred stock and fixed-maturity interests in PEPS I for other-than-temporary impairment, based on the valuation of the limited partnership interests held by PEPS I, and recognize an impairment loss if necessary. We cannot assure that we will not have to recognize impairment losses with respect to our PEPS I interests in the future.

        The FASB has a current project on its agenda that is expected to result in a change to U.S. generally accepted accounting principles with respect to financial asset transfers such as the PEPS I transaction. We cannot assure that the current accounting for our PEPS I investments will be unaffected by these possible changes.

Our net pension liabilities may continue to grow, which could adversely affect our stockholders' equity, net income, cash flow and liquidity, and require us to make additional cash contributions to the pension plans.

        To the extent that the present value of the benefits incurred to date for pension obligations in the major countries in which we operate continue to exceed the market value of the assets supporting these obligations, our financial position and results of operations may be adversely affected. Primarily as a result of the decline in the equity markets over the past several years, some of our defined benefit pension plans, particularly in the U.K., have suffered significant valuation losses in the assets backing the related pension obligation.

        Current projections indicate that our 2005 defined benefit pension expense for our major pension plans would increase by approximately $18 million compared with 2004 and that cash contributions of approximately $202 million would be required in 2005. Total cash contributions to these major defined benefit pension plans in 2004 were $189 million, a decrease of $28 million over 2003. Future estimates are based on certain assumptions, including discount rates, interest rates, fair value of assets for some of our plans and expected return on plan assets. We are currently taking actions to manage our pension liabilities, including closing certain plans to new participants. However, changes in our pension benefit obligations and the related net periodic costs or credits may occur in the future due to any variance of actual results from our assumptions and changes in the number of participating employees. As a result, there can be no assurance that we will not experience future decreases in stockholders' equity, net income, cash flow and liquidity or that we will not be required to make additional cash contributions in the future beyond those which have been announced.

We are subject to a number of contingencies and legal proceedings which, if determined unfavorably to us, would adversely affect our financial results.

        We are subject to numerous claims, lawsuits, regulatory matters and tax assessments in connection with our businesses. The damages that may be claimed are substantial, including, in many instances, claims for punitive or extraordinary damages. The litigation naming us as a defendant ordinarily involves our activities as a broker or provider of insurance products or as an employer. It is possible that, if the outcomes of these contingencies and legal proceedings were not favorable to us, it could materially adversely affect our future financial results. In addition, our results of operations, financial condition or liquidity may be adversely affected if in the future our insurance coverage proves to be inadequate or unavailable or there is an increase in liabilities for which we self-insure.

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        On April 21, 2004, Aon received a subpoena from the Office of the Attorney General of the State of New York calling for the production of documents relating to Placement Service Agreements, Market Service Agreements and similar agreements under which insurance carriers pay compensation to Aon beyond standard commissions. The office subsequently issued several other requests for information to Aon as part of its inquiry into alleged practices in the insurance industry, including bid-rigging, fictitious quotes, "tying," and "steering" of business. The departments of insurance or attorneys general of approximately 25 other states have also issued subpoenas or requested information regarding these and other issues. Aon is fully cooperating with all of these investigations.

        Purported clients have also filed civil litigation against Aon and other companies under a variety of laws and legal theories relating to broker compensation practices and other issues under investigation by New York and other states. As previously reported, a putative class action styled Daniel v. Aon (Affinity) has been pending in the Circuit Court of Cook County, Illinois since August 1999. On July 28, 2004, the Court granted plaintiff's motion for class certification. On March 9, 2005, the Court gave preliminary approval to a nationwide class action settlement within the $40 million reserve established in the fourth quarter of 2004.

        Beginning in June 2004, a number of other putative class actions have been filed against Aon and other companies by purported clients under a variety of legal theories, including state tort, contract, fiduciary duty, and statutory theories, and federal antitrust and the Racketeer Influenced and Corrupt Organizations Act theories. These actions are currently pending at early stages in state court in California and Florida and in federal court in Illinois, South Carolina and New Jersey. Aon believes it has meritorious defenses in all of these cases, and intends to vigorously defend itself against these claims. The outcomes of these lawsuits, and any losses or other payments that may occur as a result, cannot be predicted at this time.

        Beginning in late October and early November 2004, several putative securities class actions have been filed against Aon in the United States District Court for the Northern District of Illinois. Also beginning in late October and early November 2004, several putative ERISA class actions were filed against Aon in the United States District Court for the Northern District of Illinois. Aon believes it has meritorious defenses in all of these cases, and intends to vigorously defend itself against these claims. The outcomes of these lawsuits, and any losses or other payments that may occur as a result, cannot be predicted at this time.

        In early February 2005 the Company received a subpoena from the U.S. Department of Labor regarding compensation arrangements in connection with clients' employee benefit plans. The Company is cooperating with the investigation.

        Although the ultimate outcome of all matters referred to above cannot be ascertained, and liabilities in indeterminate amounts may be imposed on us, on the basis of present information, amounts already provided, availability of insurance coverages and legal advice received, it is the opinion of management that the disposition or ultimate determination of such claims will not have a material adverse effect on the consolidated financial position of Aon. However, it is possible that future results of operations or cash flows for any particular quarterly or annual period could be materially affected by an unfavorable resolution of these matters.

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Our success depends, in part, on our ability to attract and retain experienced and qualified personnel.

        Our future success depends on our ability to attract and retain experienced personnel, including underwriters, brokers and other professional personnel. Competition for such experienced professional personnel is intense. If we cannot hire and retain talented personnel, our business, operating results and financial condition could be adversely affected.

        In September 2004, we announced that we were implementing a management succession plan and began a search for a new chief executive officer. Patrick G. Ryan, our current chief executive officer and chairman of the Board of Directors, will continue to serve as chief executive officer until a successor is found.

        We are actively seeking a new chief executive officer, but we can provide no assurances as to when an executive will be hired to fill this management position. In addition, we cannot give assurance that this search and any related uncertainty regarding the future composition of our management team will not adversely impact our results of operations. Once we hire a chief executive officer, our business may be impacted by our ability to successfully integrate him or her, his or her familiarity with our business, his or her ability to develop relationships with our employees and his or her implementation of new business strategies.

We are subject to increasing costs arising from errors and omissions claims against us.

        We have experienced an increase in the frequency and severity of errors and omissions claims against us, which has and may continue to substantially increase our expenses. In our insurance brokerage business, we often assist our clients with matters which include the placement of insurance coverage and the handling of related claims. Errors and omissions claims against us may allege our potential liability for all or part of the amounts in question. Errors and omissions claims could include, for example, the failure of our employees or sub-agents, whether negligently or intentionally, to place coverage correctly or notify claims on behalf of clients or to provide insurance carriers with complete and accurate information relating to the risks being insured. It is not always possible to prevent and detect errors and omissions, and the precautions we take may not be effective in all cases. In addition, errors and omissions claims may harm our reputation or divert management resources away from operating our business.

Our businesses are subject to extensive governmental regulation which could reduce our profitability or limit our growth.

        Our businesses are subject to extensive federal, state and foreign governmental regulation and supervision, which could reduce our profitability or limit our growth by increasing the costs of regulatory compliance, limiting or restricting the products or services we sell or the methods by which we sell our products and services or subjecting our businesses to the possibility of regulatory actions or proceedings. With respect to our insurance brokerage businesses, this supervision generally includes the licensing of insurance brokers and agents and third-party administrators and the regulation of the handling and investment of client funds held in a fiduciary capacity. Our continuing ability to provide insurance brokering and third-party administration in the jurisdictions in which we currently operate depends on our compliance with the rules and regulations promulgated from time to time by the regulatory authorities in each of these jurisdictions. Also, we can be affected indirectly by the governmental regulation and supervision of other insurance companies. For instance, if we are providing managing general underwriting services for an insurer we may have to contend with regulations affecting our client. Further, regulation affecting the insurance companies with whom our brokers place business can affect how we conduct those operations.

        Most insurance regulations are designed to protect the interests of policyholders rather than stockholders and other investors. In the United States, this system of regulation, generally administered by a department of insurance in each state in which we do business, affects the way we can conduct our

13

insurance underwriting business. Furthermore, state insurance departments conduct periodic examinations of the affairs of insurance companies and require the filing of annual and other reports relating to the financial condition of insurance companies, holding company issues and other matters.

        Although the federal government does not directly regulate the insurance business, federal legislation and administrative policies in several areas, including employee benefit plan regulation, age, race, disability and sex discrimination, investment company regulation, financial services regulation, securities laws and federal taxation, do affect the insurance industry generally and our insurance underwriting subsidiaries in particular. With respect to our international operations, we are subject to various regulations relating to, among other things, licensing, currency, policy language and terms, reserves and the amount of local investment. These various regulations also add to our cost of doing business through increased compliance expenses, the financial impact of use of capital restrictions and increased training and employee expenses. Furthermore, the loss of a license in a particular jurisdiction could restrict or eliminate our ability to conduct business in that jurisdiction.

        In all jurisdictions the applicable laws and regulations are subject to amendment or interpretation by regulatory authorities. Generally, such authorities are vested with relatively broad discretion to grant, renew and revoke licenses and approvals, and to implement regulations. Accordingly, we may be precluded or temporarily suspended from carrying on some or all of our activities or otherwise fined or penalized in a given jurisdiction. No assurances can be given that our businesses can continue to be conducted in any given jurisdiction as they have been in the past.

Our significant global operations expose us to various international risks that could adversely affect our business.

        A significant portion of our operations is conducted outside the United States. Accordingly, we are subject to legal, economic and market risks associated with operating in foreign countries, including:

•

the general economic and political conditions existing in those countries;



•

imposition of limitations on conversion of foreign currencies or remittance of dividends and other payments by foreign subsidiaries;



•

imposition or increase of withholding and other taxes on remittances and other payments by subsidiaries;



•

hyperinflation in certain foreign countries;



•

imposition or increase of investment and other restrictions by foreign governments;



•

longer payment cycles;



•

greater difficulties in accounts receivables collection; and



•

the requirement of complying with a wide variety of foreign laws.

        Some of our foreign brokerage subsidiaries receive revenues in currencies that differ from their functional currencies. We must also translate the financial results of our foreign subsidiaries into United States dollars. Although we use various derivative financial instruments to help protect against adverse transaction and translation effects due to exchange rate fluctuations, we cannot eliminate such risks, and significant changes in exchange rates may adversely affect our results.

Our financial results could be adversely affected if assumptions used in establishing our underwriting reserves differ from actual experience.

        We maintain reserves as an estimate of our liability under insurance policies issued by our insurance underwriting subsidiaries. The reserves that we maintain that could cause variability in our financial results consist of (1) unearned premium reserves, (2) policy and contract claim reserves, and

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(3) future policy benefit reserves. Unearned premium reserves generally reflect our liability to return premiums we have collected under policies in the event of the lapse or cancellation of those policies. Under U.S. generally accepted accounting principles, premiums we have collected generally become "earned" over the life of a policy by means of a reduction in the amount of the unearned premium reserve associated with the policy. Unearned premium reserves are particularly significant with respect to our warranty business, given that the premiums we receive for warranty products generally cover an extended period of time. If there are significant lapses or cancellations of these types of policies, or expected losses for existing policies develop adversely and therefore premiums are not earned as expected, it may be necessary to accelerate the amortization of deferred policy acquisition expenses associated with the policies, because these deferred expenses are amortized over the projected life of the policies, or establish additional reserves to cover premium deficiencies.

        Policy and contract claim reserves reflect our estimated liability for unpaid claims and claims adjustment expenses, including legal and other fees and general expenses for administering the claims adjustment process, and for reported and unreported losses incurred as of the end of each accounting period. If the reserves originally established for future claims prove inadequate, we would be required to increase our liabilities, which could have an adverse effect on our business, results of operations and financial condition.

        The obligation for policy and contract claims does not represent an exact calculation of liability. Rather, reserves represent our best estimate of what we expect the ultimate settlement and administration of claims will cost. These estimates represent informed judgments based on our assessment of currently available data, as well as estimates of future trends in claims severity, frequency, judicial theories of liability and other factors. Many of these factors are not quantifiable in advance and both internal and external events, such as changes in claims handling procedures, inflation, judicial and legal developments and legislative changes, can cause our estimates to vary. The inherent uncertainty of estimating reserves is greater for certain types of liabilities, where the variables affecting these types of claims are subject to change and long periods of time may elapse before a definitive determination of liability is made. Reserve estimates are periodically refined as experience develops and further losses are reported and settled. Adjustments to reserves are reflected in the results of the periods in which such estimates are changed. Because setting the level of reserves for policy and contract claims is inherently uncertain, we cannot assure that our current reserves will prove adequate in light of subsequent events.

        Future policy benefit reserves generally reflect our liability to provide future life insurance benefits and future accident and health insurance benefits on guaranteed renewable and non-cancelable policies. Future policy benefit reserves on accident and health and life products have been provided on the net level premium method. These reserves are calculated based on assumptions as to investment yield, mortality, morbidity and withdrawal rates that were determined at the date of issue and provide for possible adverse deviations.

The perceived conflicts associated with our insurance brokerage and underwriting businesses could limit our growth.

        Historically, we have not been able to take advantage of certain business opportunities due to the perceived conflicts associated with owning both our insurance brokerage and underwriting businesses. For example, we have generally refrained from offering our extended warranty products and services through competing insurance brokers. Independent brokers have been reluctant to do business with our insurance underwriting business because they believed that any fees or information provided to us would ultimately benefit our competing brokerage business. These brokers also have been concerned that any information gleaned by our underwriting business regarding their clients and their clients' insurance needs would be shared with our competing brokerage business to solicit new business from these clients. Similarly, competing underwriters have feared that our brokers could share information

15

with our underwriting business in an effort to help secure desirable business or, alternatively, seek price quotes from them only for undesirable business. In the future, these perceived conflicts could limit our ability to expand our product and service offerings and seek new business through independent brokerage channels.

Each of our business lines may be adversely affected by an overall decline in economic activity.

        The demand for property and casualty insurance generally rises as the overall level of economic activity increases and generally falls as such activity decreases, affecting both the commissions and fees generated by our brokerage and consulting businesses and the premiums generated by our underwriting businesses. In particular, a growing number of insolvencies associated with an economic downturn, especially insolvencies in the insurance industry, could adversely affect our brokerage business through the loss of clients or by hampering our ability to place insurance and reinsurance business. Moreover, the results of our consulting business are generally affected by the level of business activity of our clients, which in turn is affected by the level of economic activity in the industries and markets these clients serve. As our clients become adversely affected by declining business conditions, they may choose to delay or forgo consulting engagements with us.

Recent and proposed accounting rule changes could negatively affect our financial position and results.

        Recent accounting changes effected and proposals made could negatively affect our financial position and results of operations. In December 2004, the FASB issued Statement No. 123 (revised 2004) Share-Based Payment. Statement No. 123(R) is a revision of Statement No. 123, and supersedes APB Opinion No. 25. Statement No. 123(R) establishes standards for the accounting of transactions in which an entity exchanges its equity instruments for goods or services. It also addresses transactions in which an entity incurs liabilities in exchange for goods or services that are based on the fair value of the entity's equity instruments or that may be settled by the issuance of those equity instruments.

        Currently we follows APB No. 25 for share-based payments to employees. As such, we expense the cost of stock awards over the period during which an employee is required to provide service in exchange for the award. We currently do not recognize compensation expense for stock options. Statement No. 123 (R) will require us to recognize compensation expense, beginning in the third quarter 2005, for the unvested portion of compensation expense related to stock options issued before July 1, 2005. The expense will be recognized over the remaining vesting period of the options. We will continue to recognize compensation expense for stock awards issued before July 1, 2005.

        All stock awards and options issued after July 1, 2005 will be accounted for under the rules of Statement No. 123(R). The Statement requires us to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award (with limited exceptions). The cost will then be recognized over the period during which an employee is required to provide service in exchange for the award. No compensation expense is recognized for equity instruments for which employees do not render the requisite service. The Statement allows companies to choose among various valuation techniques to estimate the fair value at the grant date of employee stock options and similar instruments.

        We are currently evaluating the Statement's transition methods and do not expect this Statement to have an effect materially different than that of the pro forma Statement No. 123 disclosures provided in Note 1 to our consolidated financial statements.

We have substantial debt outstanding that could adversely affect our financial flexibility.

        We have substantial debt outstanding. As of December 31, 2004, we had total consolidated debt outstanding, including our redeemable preferred stock, of approximately $2.2 billion. This substantial amount of debt outstanding could adversely affect our financial flexibility.

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We are a holding company and, therefore, may not be able to receive dividends in needed amounts from our subsidiaries.

        Our principal assets are the shares of capital stock of our subsidiaries, including our insurance underwriting companies. We have to rely on dividends from these subsidiaries to meet our obligations for paying principal and interest on outstanding debt obligations and for paying dividends to stockholders and corporate expenses. Payments from our underwriting subsidiaries are limited by governmental regulation and depend on the surplus and future earnings of these subsidiaries. In some circumstances, specific payments from our insurance underwriting subsidiaries may require prior regulatory approval, and we may not be able to receive dividends from these subsidiaries at times and in the amounts we anticipate or require.

The volume of premiums we write and our profitability are affected by the availability of reinsurance and the size and adequacy of our insurance company subsidiaries' capital base.

        The level of business that our insurance underwriting subsidiaries are able to write depends on the size and adequacy of their capital base. Many state insurance laws to which they are subject impose risk-based capital requirements for purposes of regulating insurer solvency. Insurers having less statutory surplus than that required by the risk-based capital model formula generally are subject to varying degrees of regulatory scrutiny and intervention depending on the level of capital inadequacy. As of December 31, 2004, each of our insurance company subsidiaries exceeds NAIC risk-based statutory surplus requirements.

        We purchase reinsurance for certain of the risks underwritten by our insurance company subsidiaries. Market conditions beyond our control determine the availability and cost of the reinsurance protection we purchase, which may affect the level of business we are able to write and our profitability. We cannot assure that we will be able to maintain our current reinsurance facilities or that we can obtain other reinsurance facilities in adequate amounts and at favorable rates. If we are unable to renew our expiring facilities or to obtain new reinsurance facilities, either our net exposures would increase or, if we are unwilling to bear an increase in net exposures, we would have to reduce the level of our underwriting commitments. Either of these potential developments could adversely affect our underwriting business.

We cannot guarantee that our reinsurers will pay in a timely fashion, if at all.

        To better manage our portfolio of underwriting risk, we purchase reinsurance by transferring part of the risk that we assume (known as ceding) to a reinsurance company in exchange for part of the premium that we receive in connection with the risk. Although reinsurance makes the reinsurer liable to us to the extent the risk is transferred (or ceded) to the reinsurer, it does not relieve us of our liability to our policyholders. Accordingly, we bear credit risk with respect to our reinsurers. Recently, due to industry and general economic conditions, there is an increasing risk of insolvency among reinsurance companies, resulting in a greater incidence of litigation and affecting the recoverability of claims. We cannot assure that our reinsurers will pay the reinsurance recoverables owed to us or that they will pay these recoverables on a timely basis.

Information Concerning Forward-looking Statements

        This report contains certain statements related to future results, or states our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include: general economic conditions in different countries in which we do business around the world, changes in global equity and fixed income markets that could affect the return on invested

17

assets, fluctuations in exchange and interest rates that could influence revenue and expense, rating agency actions that could affect our ability to borrow funds, funding of our various pension plans, changes in the competitive environment, changes in commercial property and casualty markets and commercial premium rates that could impact revenues, changes in revenues and earnings due to the elimination of contingent commissions, other uncertainties surrounding a new compensation model, the impact of regulatory investigations brought by state attorneys general and state insurance regulators related to our compensation arrangements with underwriters and related issues, the impact of class actions and individual lawsuits including derivative actions and claims under ERISA, the cost of resolution of other contingent liabilities and loss contingencies, the difference in ultimate paid claims in our underwriting companies from actuarial estimates, and other factors disclosed under "Risk Factors" elsewhere in this document.

Website Access to Reports and Other Information

        Our annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K and all amendments to those reports are made available free of charge through our website (http://www.aon.com) as soon as practicable after such material is electronically filed with or furnished to the Securities and Exchange Commission. Also posted on our website, and available in print upon request, are the charters for our Audit Committee, Organization and Compensation Committee, Governance/Nominating Committee and Investment Committee, our Governance Guidelines, our Code of Ethics and our Code of Ethics for Senior Financial Officers. Within the time period required by the SEC and the New York Stock Exchange, we will post on our website any amendment to or waiver of the Code of Ethics for Senior Financial Officers, as well as any amendment to the Code of Ethics or waiver thereto applicable to any executive officer or director. The information provided on our website is not part of this report, and is therefore not incorporated herein by reference.

Item 2.    Properties.

        Our business activities are conducted principally in leased office space in cities throughout the world. Certain of our subsidiaries do own and occupy a few office buildings in the U.S. and certain foreign countries. In general, no difficulty is anticipated in negotiating renewals as leases expire or in finding other satisfactory space if the premises become unavailable. In certain circumstances, we may have unused space and may seek to sublet such space to third parties, depending upon the demands for office space in the locations involved.

Item 3.    Legal Proceedings.

        We hereby incorporate by reference Note 14, "Contingencies," of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

Item 4.    Submission of Matters to a Vote of Security Holders.

        None.

Executive Officers of the Registrant

        Our executive officers are regularly elected by our Board of Directors at the annual meeting of the Board which is held following each annual meeting of our stockholders. Our executive officers were elected to their current positions on May 21, 2004 to serve until the meeting of the Board following the

18

annual meeting of stockholders to be held on May 20, 2005. Ages shown for executive officers are as of December 31, 2004.

        Information concerning Mr. Ryan is incorporated by reference from the disclosure set forth under the heading "Election of Directors" in our Proxy Statement for the 2005 Annual Meeting of Stockholders to be held on May 20, 2005.

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PART II

Item 5.    Market for Registrant's Common Equity, Related Stockholder Matters and Issuer Repurchases of Equity Securities.

        Aon's common stock, par value $1.00 per share, is traded on the New York Stock Exchange. We hereby incorporate by reference the "Dividends paid per share" and "Price range" data under the heading "Quarterly Financial Data" in Part II, Item 8 of this report.

        Aon had approximately 11,231 holders of record of its common stock as of February 28, 2005.

        We hereby incorporate by reference Note 10, "Redeemable Preferred Stock, Capital Securities and Stockholders' Equity" of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

        During the fourth quarter ended December 31, 2004, no purchases of Aon's common stock were made by or on behalf of Aon or any "affiliated purchaser" (as defined in Rule 10b-18(a)(3) under the Securities Exchange Act of 1934).

        Information relating to the compensation plans under which equity securities of Aon are authorized for issuance is set forth under Part III, Item 12 of this report and is incorporated herein by reference.

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Item 6.    Selected Financial Data.

Selected Financial Data

(1)

In first quarter 2002, Aon adopted FASB Statement No. 142, Goodwill and Other Intangible Assets, effective January 1, 2002. Beginning in 2002, amortization of goodwill is no longer included in net income.

(2)

Adoption of SEC Staff Accounting Bulletin 101, effective January 1, 2000, net of tax.

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Item 7.    Management's Discussion and Analysis of Financial Condition and Results of Operations.

        This Management's Discussion and Analysis is organized as follows:

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OVERVIEW

Key Drivers of Financial Performance

Segments

        The key drivers of financial performance vary among our segments.

        Risk and Insurance Brokerage Services.    Brokerage segment results are affected by a number of key drivers, including (i) conditions in insurance markets generally (particularly fluctuations in premiums charged by insurance companies), (ii) success in attracting new clients and avoiding loss of existing clients, (iii) managing our expenses, (iv) fluctuations in foreign exchange rates and (v) interest income on our investments. In addition, in 2004, this segment was substantially affected by the matters discussed under "Key Recent Events—Investigation by the New York Attorney General (AG) and Other Regulatory Authorities" and our related decision to terminate contingent commission arrangements. In connection with the elimination of contingent commission arrangements, we are in the process of establishing a new business compensation model.

        Consulting.    Consulting segment results are principally affected by (i) the employment levels of our clients that are mainly driven by economic conditions, (ii) governmental regulations affecting the health care market, employee benefit programs, and our clients' respective industries, (iii) success in attracting new clients and retaining existing clients, (iv) our success in cross selling services among business units, and (v) managing our overall level of expenses. In addition, in 2004, this segment was also affected by the matters discussed under "Key Recent Events—Investigation by the New York Attorney General and Other Regulatory Authorities" and our related decision to terminate contingent commission arrangements.

        Insurance Underwriting.    Underwriting segment revenues are affected by (i) consumer buying habits that are influenced by economic conditions, (ii) our assumption of select commercial property and casualty insurance business particularly from our managing underwriting group in our Risk and Insurance Brokerage Services segment, (iii) competition with other underwriters (including competition based upon claims-paying ratings), (iv) success in selling new policies, selling existing policyholders more services, and having customers renew their policies, (v) the effectiveness and collectability of our reinsurance contracts, particularly in programs where we serve as the fronting company, ceding substantially all risk, and (vi) investment results.

        Corporate and Other.    The key drivers of results in this segment are investment income and debt financing costs.

Liquidity

        Liquidity is derived from cash flows from our business, excluding funds held on behalf of clients, and from financing. We use liquidity for capital expenditures, to repay debt, to fund acquisitions and pension obligations, and to pay dividends to our stockholders. Because we are a holding company, our subsidiaries may not have available cash to pay us dividends (which, in the case of the insurance underwriting subsidiaries, is limited by regulatory and rating agency considerations). Tax considerations may affect access to cash generated from operations outside the United States, as can pension funding requirements in both our domestic and international pension plans.

Executive Summary of 2004 Financial Results

        We are proud of the high quality of our products and services, the breadth and depth of our intellectual capital, and the leading market positions that we have built.

        In 2004, consolidated revenues from continuing operations increased 5% to $10.2 billion, mainly due to the weakening of the U.S. dollar against foreign currencies. Our organic revenue growth (which adjusts revenue growth for the effects of foreign exchange and other factors) was flat for the year.

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        Our income from continuing operations before income taxes and minority interest fell $251 million from 2003. The decrease in income was primarily from our Risk and Insurance Brokerage Services segment, as well as an increase in interest expense of $58 million due to the adoption at December 31, 2003 of FIN 46.

        Several factors that hurt our 2004 results included:

•

a $180 million provision for settlements with the New York AG and other regulatory authorities and a $40 million provision for costs and payments to settle the Daniel class action lawsuit,



•

increased net periodic pension expense related to our major defined benefit pension plans of $47 million, which primarily affected our Risk and Insurance Brokerage Services segment,



•

lost contingent commission revenue of approximately $47 million in 2004 due to the termination of these arrangements as of October 1, 2004, and



•

an $80 million unrealized investment gain recognized in earnings in 2003 relating to our warrants in Endurance Specialty Holdings, Inc. ("Endurance"). The value of these warrants in 2004 remained consistent with the December 31, 2003 value. Partially offsetting the absence of an unrealized gain on the warrants in 2004 were realized gains of $48 million from the sale of most of our common stock investment in Endurance.

        We are working to improve our margins by seeking organic revenue growth and greater financial discipline. More specifically, we are:

•

working to prudently manage employee compensation and benefit expenses;



•

in light of our terminating contingent commission arrangements, working with our clients, insurance carriers, regulators and others to establish a new business model that will ensure that we are properly compensated, while maintaining transparency and the trust of our clients;



•

reviewing strategic alternatives for our various businesses. During 2004, we sold our U.S. and U.K. claims businesses, as well as other small operations;



•

leveraging our purchasing power with vendors, suppliers, and landlords;



•

pursuing alternative resourcing strategies, such as outsourcing, to more efficiently provide non-client-facing services. In third quarter 2004, we began outsourcing most of our U.S. information technology infrastructure to Computer Sciences Corporation. Over the seven-year agreement, we expect to realize approximately $300 million of aggregate savings beginning in the second half of 2005 from outsourcing and consolidating data centers and other functions; and



•

offering additional services to existing clients who can benefit from our wide range of resources.

        We continue to do a better job of generating and managing our cash. More specifically, we:

•

grew our cash and investment balances. Our cash balance increased $30 million and our total investment balance increased $1.3 billion;



•

paid down $46 million of total debt in 2004. We paid down $305 million of domestic long-term debt that was scheduled to be redeemed this year. That decrease was mostly offset by an increase in borrowings at our foreign subsidiaries. We drew down $334 million on our long-term Euro credit facility to ensure adequate liquidity in the fourth quarter of 2004. Our debt and preferred stock to total capital percentage declined from 32.8% at December 31, 2003 to 29.8% at December 31, 2004; and



•

reduced capital expenditure spending by $105 million or 57% from 2003.

        All of Aon's financial information reflects the application of critical accounting policies, estimates, assumptions and judgments, as discussed below under "Critical Accounting Policies and Estimates."

        Further discussion of these items may be found in the remainder of this Management's Discussion and Analysis.

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KEY RECENT EVENTS

Investigation by the New York Attorney General and Other Regulatory Authorities

        The insurance industry has recently come under significant scrutiny by various regulatory authorities.

        In April 2004, the New York AG began investigating various insurance industry practices, including placement service agreements, market service agreements, and similar agreements under which insurance carriers pay compensation to insurance brokers, including Aon, beyond standard commissions. The New York AG issued subpoenas to various companies in the insurance industry, including Aon, related to these agreements and various other practices, including alleged tying of reinsurance, bid rigging, and soliciting fictitious quotes. Other state attorneys general and state departments of insurance have also issued subpoenas to Aon or begun investigations into contingent commissions and other business practices of brokers, agents and insurers, and some state regulators have announced that they intend to enact new regulations or policies to govern these practices. Contingent commissions generally are non-service-specific, volume- or profit-based compensation arrangements between insurers and brokers. Similarly, regulatory authorities in other countries are either considering or have already begun similar inquiries. Aon is fully cooperating with all the investigations, and has retained outside counsel to conduct its own internal review of its compensation and other practices.

        In October 2004, the New York AG filed a complaint against Marsh & McLennan Company, Inc., and its subsidiary, Marsh Inc., alleging that Marsh committed fraud and violated New York State antitrust and securities' laws. On October 15, 2004, Marsh announced that it was suspending the use of contingent commission agreements.

        On October 22, 2004, we announced that we were terminating contingent commission arrangements with underwriters. We have nearly completed this process and are working with clients, insurance carriers, regulators, and others to establish a new business model that ensures that we link compensation to specific, measurable services in a way that is transparent, easy to understand, and accepted by clients. Other insurance brokers and carriers have also announced that they will terminate contingent commission arrangements.

        For the year ended December 31, 2004, we earned approximately $132 million of contingent commissions versus $169 million in 2003. Of the $132 million contingent commissions earned in 2004:

•

$111 million was included in our Risk and Insurance Brokerage Services segment and



•

$21 million was included in our Consulting segment.

        We earned $15 million of contingent commissions in fourth quarter 2004 versus $52 million in 2003. The amount recorded in this year's fourth quarter represents amounts earned on arrangements covering periods prior to October 1, 2004. A small number of these arrangements call for the calculations to be performed on an annual basis. Some of these calculations occurred in the fourth quarter, and minimal amounts are likely to be recorded in 2005.

        Contingent commission revenue was $37 million lower in 2004 compared to 2003 as a direct result of terminating our contingent commission arrangements. However, we estimated $47 million of lost contingent commission revenue in 2004 that would have been earned had we not terminated these arrangements. The loss of revenues from these agreements will likely have a material adverse effect on our results of operations for 2005. As of December 31, 2004, we have approximately $50 million of net receivables recorded related to contingent commissions, which we believe we are entitled to and will collect.

        During 2004, we also earned approximately $143 million of other compensation for services to underwriters, all of which is included in our Risk and Insurance Brokerage Services segment. This other compensation encompasses activities such as affinity program management, managing general

25

underwriting, and wholesale brokerage where we act as the agent for carriers. In addition, this includes compensation for specific services as is customary in some markets outside the United States.

        On March 4, 2005, Aon Corporation ("Aon") and its subsidiaries and affiliates (collectively, the "Company") entered into an agreement (the "Settlement Agreement") with the Attorney General of the State of New York, the Superintendent of Insurance of the State of New York, the Attorney General of the State of Connecticut, the Illinois Attorney General and the Director of the Division of Insurance, Illinois Department of Financial and Professional Regulation (collectively, the "State Agencies") to resolve all the issues related to investigations conducted by the State Agencies.

        The material terms of the Settlement Agreement are as follows:

        The Company will pay $190 million into a fund (the "Fund") to be distributed to certain eligible policyholder clients. These payments are in full satisfaction of the Company's obligations under the Settlement Agreement and the State Agencies have agreed not to impose any other financial obligation or liability on the Company related to the lawsuits. No portion of the payments by the Company is considered a fine or penalty. The Company will make payments into the Fund as follows:

•

On or before September 1, 2005, the Company shall pay $76 million into the Fund.



•

On or before September 1, 2006, the Company shall pay $76 million into the Fund.



•

On or before September 1, 2007, the Company shall pay $38 million into the Fund.

        The Fund, plus interest, will be used to compensate the Company's eligible policyholder clients according to procedures set out in the Settlement Agreement. No amount paid to the Fund will be returned to Aon under any circumstances.

        On or before June 30, 2005, the Company will calculate, in accordance with a formula approved by the State Agencies, the amount that each policyholder client is eligible to receive from the Fund. Clients eligible to participate in the Fund are those U.S. clients that engaged the Company to place, renew, consult on or service insurance with inception or renewal dates between January 1, 2001 through December 31, 2004 (the "Relevant Period") where such placement, renewal, consultation or servicing resulted in contingent commissions or overrides recorded by Aon during the Relevant Period (the "Eligible Policyholders").

        On or before June 30, 2005, the Company must send a notice to each Eligible Policyholder setting forth, among other things, the amount it will be paid from the Fund if it elects to participate (a "Participating Policyholder"). Participating Policyholders must tender a release of claims against the Company arising from acts, omissions, transactions or conduct that are the subject of the lawsuits.

        On November 30, 2005, September 30, 2006 and September 30, 2007, each Participating Policyholder shall receive from the Fund as much of that Participating Policyholder's aggregate share of the Fund as possible with the monies then available in the Fund.

        In the event that an Eligible Policyholder elects not to participate or otherwise does not respond by October 30, 2005 (a "Non-Participating Policyholder"), that client's allocated share may be used by the Company to satisfy any pending or other claims asserted by clients relating to issues in the Settlement Agreement. In no event shall a distribution be made from the Fund to any other client until all Participating Policyholders have been paid, nor shall total payments to any Non-Participating Policyholder exceed 80% of that policyholder's original allocated share. If any funds remain in the Fund as of October 1, 2007 such funds shall be distributed pro rata to the Participating Policyholders by November 1, 2007. In no event shall any of the amounts paid into the Fund be used to pay attorneys' fees.

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        Within 60 days of the date of the Settlement Agreement, the Company shall commence the implementation of certain business reforms, including the following:

•

To accept only a specific fee to be paid by the client, a specific percentage commission on premium to be paid by an insurer set at the time of purchase, renewal, placement or servicing of an insurance policy, or a combination of both.



•

To fully disclose in plain, unambiguous written language commissions in either dollars or percentage amounts.



•

Not to accept any other valuable compensation or consideration from an insurer other than as stated above, including contingent compensation and any compensation or preference in connection with the selection of insurers from which to solicit bids for clients.



•

Not to request or accept from any insurer any false, fictitious or inflated quote, or quote that does not represent the insurer's best evaluation at the time of the minimum premium the insurer would require to bind the insurance coverage sought by the client.



•

Not to request or accept from any insurer any promise or commitment for the use of our services, including reinsurance brokerage, conditioned upon any arrangement to provide preferential treatment for any insurer.



•

Not to place, renew or service a client's business through a wholesale broker unless agreed to by the client after full disclosure of all the compensation to be received, any interest we may have in the wholesale broker, and any alternative to using the wholesaler broker.



•

To fully disclose to each client all quotes received in connection with coverage of the client's risk with all terms and, all commissions to be received for each quote, and to provide disclosure of and obtain clients written consent to all compensation arrangements.



•

To disclose to each client at the end of each year all compensation received during the preceding year from any insurer or third party in connection with the client's policy.



•

To implement company-wide written standards of conduct regarding compensation from insurers consistent with the terms of the settlement and institute appropriate training of employees, including business ethics, professional obligations, conflicts of interest, antitrust and trade practices compliance and record keeping.



•

To establish a Compliance Committee of our Board of Directors that will monitor our compliance with the standards of conduct regarding compensation.



•

To maintain a record of all complaints regarding compensation from any insurer, and provide such record to the Compliance Committee.



•

To file annual reports with New York and Illinois for five years.

        The Company shall not, directly or indirectly, seek or accept indemnification pursuant to any insurance policy or other reimbursement with respect to any amounts payable under the Settlement Agreement.

        In accordance with APB Opinion No. 21, Interest on Receivables and Payables, we have discounted the payment stream associated with the settlement and recorded the present value of the liability and corresponding expense of $180 million in our financial statements as of December 31, 2004. The discount was determined using our incremental borrowing rate. We have not discounted the payment due on September 1, 2005. The settlement was considered fully tax deductible and is not treated as a permanent difference in our tax calculation.

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        The difference between our agreed settlement amount of $190 million and the amount in our financial statements as of December 31, 2004 represents the discount. The discount will be amortized over the course of the payment timeframe resulting in an increase in expense.

Reconciliation between cash paid and expense incurred

        Of the $180 million expensed in 2004, $153 million was allocated to the Risk and Insurance Brokerage Services segment and the remaining $27 million was allocated to the Consulting segment.

        In addition to the New York AG and other regulatory investigations, we are defending various client class action lawsuits. We have provided $40 million for costs and payments to settle the Daniel class action lawsuit. Of the $40 million recorded, $34 million was allocated to our Risk and Insurance Brokerage segment and the remaining $6 million was allocated to our Consulting segment.

Investment in Endurance Common Stock and Warrants

        During 2004, we sold virtually all of our common stock investment in Endurance resulting in a $48 million pretax gain for the year ended December 31, 2004. We sold:

•

9.8 million shares at a net realized price of $32.70 per share, resulting in proceeds of $320 million during fourth quarter 2004 and



•

1.4 million shares at a net realized price of $33.20 per share, resulting in proceeds of $47 million during first quarter 2004.

        We invested all the proceeds in short-term investments that are held by our insurance underwriting subsidiaries.

        We retain 4.1 million stock purchase warrants in Endurance. These warrants meet the definition of a derivative as described in FASB Statement No. 133, Accounting for Derivative Instruments and Hedging Activities, which requires them to be recorded in the consolidated financial statements at fair value, with changes in fair value recognized in earnings.

        Through December 31, 2002, these warrants were carried at zero value, which approximated their original cost. In 2003, Endurance completed its initial public offering, which provided a market value for the underlying shares and removed much of the uncertainty about the fair value of Endurance and the warrants. At December 31, 2004, we determined that the warrants had a fair value of approximately $80 million, consistent with the valuation as of December 31, 2003.

        We recognized these changes in value in investment income in the Corporate and Other segment. For the year ended December 31, 2004 the total change was immaterial. The future value of the warrants may vary considerably from the value at December 31, 2004, due to the price movement of the underlying shares, the passage of time, and changes in other factors used in the valuation model (see Note 1 to the consolidated financial statements for additional information about the valuation of the warrants).

Sale of Certain Businesses

        During 2003 and 2004, we sold certain businesses, including our:

•

automotive finance servicing business, which had been in run-off since first quarter 2001,

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•

U.K. claims services businesses,



•

a small non-core consulting subsidiary,



•

U.K. reinsurance brokerage runoff unit, and



•

a small U.S. brokerage unit.

        The operating results of all of these businesses are classified as discontinued operations, and prior year's operating results have been reclassified to discontinued operations, as this table shows:

        See Note 5 to the consolidated financial statements, "Disposal of Operations," for further information.

        In November 2004, we sold our Cambridge Integrated Services Group, Inc. ("Cambridge") claims administration business to Scandent Holdings Mauritius Limited (SHM) for $90 million in cash plus convertible preferred stock in SHM, valued at $15 million. Because of our convertible preferred stock holding and other factors, we included Cambridge's results prior to the sale's effective date, as well as a pretax gain on the sale of approximately $15 million, in income from continuing operations.

        From time to time, we explore strategic alternatives for our various businesses. In February 2005, we announced that we are exploring alternatives relating to our ownership of Swett & Crawford, which is currently the largest U.S.-based wholesale insurance broker with more than 900 employees in 40 offices. By exploring alternatives, we expect to determine if Swett & Crawford's potential can be realized more fully under different ownership. No decision has been made to sell Swett & Crawford at this time.

Critical Accounting Policies and Estimates

        Aon's consolidated financial statements have been prepared according to U.S. generally accepted accounting principles (GAAP). To prepare these financial statements, we made estimates, assumptions, and judgments that affect:

•

what we report as our assets and liabilities,



•

what we disclose as contingent assets and liabilities at the date of the financial statements, and



•

the reported amounts of revenues and expenses during the periods presented.

        In accordance with our policies, we:

•

regularly evaluate our estimates, assumptions and judgments, including those concerning revenue recognition, investments, intangible assets, income taxes, financing operations, policy liabilities (including future policy benefit reserves, unearned premium reserves and policy and contract claim reserves), restructuring costs, retirement benefits, and contingencies and litigation, and



•

base our estimates, assumptions, and judgments on our historical experience and on factors we believe reasonable under the circumstances.

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        The results involve judgments about the carrying values of assets and liabilities not readily apparent from other sources. If our assumptions or conditions change, the actual results we report may differ from these estimates.

        We believe the following critical accounting policies affect the more significant estimates, assumptions and judgments we used to prepare these consolidated financial statements.

Pensions

U.S. Plans

        Effective January 1, 2004, the U.S. pension plans were closed to new employees. All new employees participate in a defined contribution plan. Over time, this change will reduce the volatility inherent in the accounting for the U.S. pension plans.

        Aon uses a market-related valuation of assets to calculate pension expense. This valuation reflects a five-year average of the difference between the expected return on plan assets and the actual market value return. The prior year market-related value is projected to the current date by adjusting for contributions, benefit payments and expected returns. The asset gain or loss is the difference between the expected return on assets and the actual return on assets. Twenty percent of the asset gain or loss is recognized in the current year's market-related value, with the remaining eighty percent spread evenly over the next four years.

        As of year-end 2004, the market-related value of assets does not yet reflect our accumulated asset losses of $142 million. These losses will increase pension expense as they are graded into the market-related asset value and may be offset by future asset gains. As of year-end 2004, we reported a fair value of pension assets of $969 million, while the market-related value of assets is $1,111 million.

        Under FASB Statement No. 87, the full gain or loss on assets and obligations is not recorded as expense in the current period. Statement No. 87 allows changes in the projected benefit obligation and market value of assets to be deferred and amortized as a component of pension expense over several years, based on the average expected future service of active employees, which is currently nine years. Gains and losses on pension obligations include the net effects of changes in the discount rate as well as demographic changes in the employee data.

        For the 2004 valuation year, the pension plans have a combined deferred loss of $584 million (comprised of unrecognized asset losses of $142 million and other than deferred asset losses of $442 million) that has not yet been recognized through income in the financial statements. We amortize the other than deferred asset losses of $442 million outside of a corridor, over about nine years; this corridor is defined as 10% of the greater of the market-related value of plan assets or the projected benefit obligation. For 2005, the estimated amortization amount to be recognized in expense is projected to be $34 million. To the extent not offset by future gains, the incremental amortization as calculated above will continue to affect future pension expense in a similar manner until fully amortized.

        The pension plan investment policy allows assets to be allocated to various asset classes. According to the policy, the percentage of total assets invested in each class should fall within a range. There is a target allocation for long-term investment decisions. However, the range provides flexibility to accommodate prevailing market conditions. In order to determine the expected long-term rate of return for the pension plan, we analyze the historical performance, investment community forecasts, and current market conditions to develop expected returns for each of the plan's asset classes. In setting the individual asset assumptions, we weight the historical performance data series most heavily toward the geometric average returns. We then weight the expected returns for each asset class by the plan's target allocation. To determine pension expense, we currently assume a long-term rate of return of 8.5%.

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        This table shows the result of the calculation based on the target asset allocation for year-end 2004. The actual return for the 2004 valuation year (10.5%) was in excess of the assumed return.

        There are several assumptions that impact the actuarial calculation of pension obligations and, in turn, net periodic pension expense in accordance with Statement No. 87. These assumptions require various degrees of judgment. The most significant assumptions are:

•

the expected return on plan assets and



•

the discount rate.

        The same assumptions are used for our pension plans and postretirement benefit plans where applicable. Changes in these assumptions can have a material impact on pension obligations and pension expense. For example, holding all other assumptions constant, a one percentage point:

•

decrease in our estimated discount rate would increase our estimated 2005 pension expense by approximately $38 million and the estimated 2005 postretirement medical benefit expense by $0.5 million and



•

increase in our estimated discount rate would decrease the 2005 estimated pension expense by approximately $31 million and the postretirement medical benefit expense by approximately $0.5 million.

        Similarly, holding other assumptions constant, a one percentage point:

•

decrease in our estimated long-term rate of return on plan assets would increase the estimated 2005 pension expense by approximately $11 million and



•

increase in the estimated long-term rate of return on plan assets would decrease pension expense by approximately $11 million.

        Required cash contributions are also sensitive to assumptions, however the assumptions used to determine contributions to the plan are changed infrequently. We anticipate cash funding requirements of $47 million in 2005 and $100 million in 2006.

Major U.K. Plans

        During 1999, the U.K. pension plans were closed to new employees, and all new employees became participants in a defined contribution plan. As with the U.S. plans, this change will gradually reduce the volatility of the accounting for U.K. pension plans. As with our other international plans, the U.K. plans are solely obligations of subsidiaries of Aon Corporation.

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        For the 2004 valuation year, the major U.K. pension plans have a combined deferred loss (from asset and liability experience) of $1,500 million that has not yet been recognized through income in the financial statements. We amortize the accumulated loss outside of a corridor over 17 years; this corridor is defined as 10% of the greater of the fair value of plan assets or the projected benefit obligation. For 2005, the estimated amortization amount to be recognized in expense is $67 million. To the extent not offset by future gains, the incremental amortization as calculated above will continue to affect future pension expense in a similar manner until fully amortized.

        To determine pension expense, we use the fair market value of plan assets. Generally, the trustees of the U.K. plans determine the investment policy for each plan. In total, at the end of the 2004 valuation year, the plans were invested 66% in equities and 34% in fixed income securities with a fair value of $2,355 million. In determining the expected rate of return, investment community forecasts and current market conditions are analyzed to develop expected returns for each of the asset classes used by the plans. Consideration is given to historic performance data over long periods in order to check the assumption in each class relative to each other. The expected returns for each asset class are then weighted by the actual asset allocation of the plans. To determine pension expense, we currently assume a long-term rate of return of 7.25%.

        This table shows the result of the calculation based on the actual asset allocation for year-end 2004. The actual return for the 2004 valuation year (8.2%) was in excess of the assumed return.

        With respect to U.K. pension liabilities, a one-percentage point:

•

decrease in our estimated discount rate would increase the estimated 2005 pension expense by approximately $51 million and



•

increase in our estimated discount rate would decrease the estimated 2005 pension expense by approximately $48 million.

        Similarly, a one-percentage point:

•

decrease in our estimated long-term rate of return on plan assets would increase the estimated 2005 pension expense by approximately $23 million and



•

increase in our estimated long-term rate of return on plan assets would decrease estimated 2005 pension expense by approximately $23 million.

        Cash flow requirements are also sensitive to assumptions, however the assumptions used for funding the U.K. plans are changed infrequently. Under current rules and assumptions, we anticipate U.K. funding requirements of $152 million in both 2005 and 2006. These contributions reflect minimum funding requirements plus other amounts agreed to with the trustees of the U.K. plans.

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Dutch Plan

        To determine pension expense, we use the fair market value of plan assets that, at year-end 2004, amounted to $363 million. At the end of 2004, the Dutch pension plan has a combined deferred loss of $135 million that has not yet been recognized through income in the financial statements. We amortize the accumulated loss outside of a corridor over 20 years; this corridor is defined as 10% of the greater of the fair value of plan assets or the projected benefit obligation. For 2005, the estimated amortization amount to be recognized in expense is $5 million. To the extent not offset by future gains, the incremental amortization as calculated above will continue to affect future pension expense in a similar manner until fully amortized.

        The target asset allocation is 35% equities and 65% fixed income securities, with an allowed deviation of 5%. At year-end 2004, the actual asset allocation was consistent with the target allocation. The expected long-term rate of return is 6%, which results from an expected future return of 8% on equities and a 5% return on fixed income investments.

        With respect to Dutch pension liabilities, a one percentage point:

•

decrease in our estimated discount rate would increase the estimated 2005 pension expense by approximately $6 million and



•

increase in our estimated discount rate would decrease the estimated 2005 pension expense by approximately $5 million.

        A one percentage point:

•

decrease in our estimated long-term rate of return on plan assets would increase the estimated 2005 pension expense by approximately $4 million and



•

increase in our estimated long-term rate of return on plan assets would decrease the estimated 2005 pension expense by approximately $4 million.

        At year-end 2004, the Dutch pension plan had a prepaid pension asset of $121 million. In the future, if the funded status of the plan deteriorates, this amount could be reflected in a minimum pension liability, which would reduce stockholders' equity. The Company accelerated a planned 2005 contribution of approximately $18 million into the pension plan before December 31, 2004.

Contingencies

        We define a contingency as any material condition that involves a degree of uncertainty that will ultimately be resolved. Under GAAP, we are required to establish reserves for contingencies when a loss is probable and we can reasonably estimate its financial impact.

        We are required to assess the likelihood of material adverse judgments or outcomes as well as potential ranges or probability of losses. We determine the amount of reserves required, if any, for contingencies after carefully analyzing each individual issue. The required reserves may change due to new developments in each issue, or changes in approach, such as changing our settlement strategy.

        We have reflected the impact of the March 2005 New York AG and other regulatory authorities settlement in our December 31, 2004 financial statements. The settlement amount is fixed and determinable, therefore the liability was discounted to the net present value of the payments based on our incremental borrowing rate in accordance with APB 21. The $10 million difference between the settlement amount of $190 million and the $180 million liability recorded in our balance sheet as of December 31, 2004 represents the discount. This discount will increase our expenses in future years as it is amortized into expense.

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        In addition to the New York AG and other regulatory authorities settlement accrual, we have also accrued $40 million for costs and payments to settle the Daniel class action lawsuit.

Policy Liabilities

        Through our insurance underwriting operations, we collect premiums from policyholders, and we establish liabilities (reserves) to pay benefits to policyholders. The liabilities for policy benefits, claims, and unearned premiums are a large portion of the total policy liabilities shown on our balance sheet, and are comprised primarily of estimated future payments to policyholders, policy and contract claims, and unearned and advance premiums and contract fees.

Accident, Health & Life

        To establish policy liabilities, we develop estimates of reported and anticipated claims, based on our historical experience, other actuarial data, and assumptions on investment yields. We base interest rate assumptions on factors such as market conditions and expected investment returns. Although mortality, morbidity, persistency, and interest rate assumptions are set when we issue new insurance policies, we may need to provide for additional losses on a product by increasing reserves, reducing previously capitalized acquisition costs established for that product, or by establishing premium deficiency reserves if there are significant changes in our experience or assumptions. The process of estimating and establishing policy and contract liabilities is inherently uncertain and the actual ultimate cost of a claim may vary materially from the estimated amount reserved.

        Liabilities for incurred but unpaid claims include estimated costs relating to incurred and reported claims and incurred but not reported claims. The liability for unpaid claims is based on the estimated ultimate cost of settling claims using best estimates of past experience. These estimates reflect known current trends and any other factors that would influence historical data. Actual experience may vary from anticipated levels due to changes in claim reporting, processing patterns and variations from historic averages for the amount paid per claim. Variations from historic patterns and averages could result in additional changes that increase or decrease unpaid claim liabilities. As of December 31, 2004, there were no known changes in reporting or processing patterns.

        Except for products that meet the definition of FASB Statement No. 97 Accounting and Reporting by Insurance Enterprises for Certain Long-Duration Contracts and for Realized Gains and Losses from the Sale of Investments, the liability for future policy benefits relating to long-duration contracts is accrued when premium revenue is recognized. The liability represents the present value of future benefits to be paid to policyholders less the present value of future premiums, and is estimated using methods that include assumptions such as estimates of expected investment yields, mortality, morbidity, and policy persistency. Emerging trends in morbidity, mortality, persistency, and asset yields may cause the actual experience to vary from original estimates. The financial impacts of changes from original assumptions are taken into account as the actual experience is realized. Some of these trends can fluctuate significantly over time. To the extent that current estimates of the present value of future benefits exceed the present value of future premiums for a product line, all excess amounts have been taken into account as a loss. There are no current estimates of the overall net gain resulting from improvements from original assumptions.

        Long-duration contracts that meet the definition of Statement No. 97, such as universal life type products are accounted for in a manner consistent with the accounting for interest-bearing or other financial instruments. Payments received on those contracts are not reported as revenue and a corresponding policy benefit reserve is not established. The liability for policy benefits is equal to the balance that accrues to the benefit of policyholders at the date of the financial statements, amounts that have been assessed to compensate the insurer for services to be performed over future periods,

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and amounts previously assessed against policyholders that are refundable on termination of the contract.

Claim Liability (millions)
Reserves for claim liabilities as of the year ended were as follows:

        A 1% increase in the assumed medical cost trends would reduce pretax income by approximately $1.8 million.

Future Policy Benefits (millions)
    Reserves for future policy benefits as of the year ended were as follows:

        If a 1% unfavorable change were to occur in the mortality and morbidity assumptions for both the accident & health and life books of business, pretax income would be decreased by approximately $7 million.

Warranty

        Our warranty business is unique in comparison to traditional underwriting businesses in that we are providing warranties for consumer goods and credit. These programs are generally reviewed by policy period to determine the necessary reserves for warranty claims. For automobile warranty, terms may extend as far as seven years with a few warranties lasting longer. Other warranty lines have terms extending up ten years with a few warranties lasting longer.

        In addition to the term of the warranty, other characteristics are taken into account when estimating the reserves. Considerations such as the manufacturer or classes of products are reviewed and embedded in our calculation methodology.

        Similar to other underwriting activities, historic loss development factors are used to project the ultimate loss. For recent periods we use the Bornhuetter-Ferguson method. This method is commonly used in underwriting businesses. The Bornhuetter-Ferguson method combines loss development methods with an expected loss ratio technique. The expected loss ratio is computed using either judgment, recent experience, or other commonly used statistical methods such as the Cape Cod method. These methods result in a point estimate of our liability, which was $822 million as of December 31, 2004. We believe that the ultimate development of the recorded liability could be as much as 10% more or 5% less.

        Sources of uncertainty include technological innovations such as plasma TVs and liquid crystal displays. In addition, some of our policies include profit sharing where the client participates in underwriting profits but we pay all underwriting losses.

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Property & Casualty

        The loss reserve estimates for all property and casualty lines of business are derived by accident year from a minimum of five standard actuarial techniques including, but not limited to, incurred and paid loss development factors based on both program history and industry development patterns for similar lines of business. A Bornhuetter-Ferguson Method is also employed incorporating historical loss ratio performance weighted with case emergence to date. Where such data is available, frequency and severity methods are used taking into account claim count emergence and severity trends.

        The selected ultimate loss estimates are based on the range of estimates discussed above. A typical selection is the average of the estimates, but that selection may also be influenced by the consistency of the estimates, knowledge of emerging loss trends, and rate or benefit changes.

        Selected ultimate losses are evaluated for business on a direct, assumed, ceded and net basis. From the selected ultimate losses, paid losses are deducted to arrive at the total reserve. The total reserve includes case reserves and incurred but not reported reserves.

        At December 31, 2004, our recorded liability was $277 million. Given the current knowledge of the overall variability of property and casualty exposures, loss reserves are expected to fall within 10 loss ratio points (or approximately $20 million) of our selected estimate 95% of the time.

Valuation of Investments

        We periodically review securities with material unrealized losses and evaluate them for other than temporary impairment. We analyze various risk factors and determine if any specific asset impairment exists. If there is a specific asset impairment, we recognize a realized loss and adjust the cost basis of the impaired asset to its fair value.

        We review invested assets with material unrealized losses each quarter. Those assets are separated into two categories:

(1)

Assets with unrealized losses due to issuer-specific events, which are segmented among four categories: fixed-maturity investments; publicly-traded preferred stocks; publicly-traded common stocks; and private common and preferred stocks and other invested assets.

(2)

Assets with unrealized losses due to market conditions or industry-related events.

Assets with unrealized losses due to issuer-specific events

Fixed maturity investments.

        At least quarterly, we:

•

review the creditworthiness of corporate obligors for changes by nationally recognized credit rating agencies and changes in fundamental financial performance of the underlying entity,



•

monitor cash flow trends and underlying levels of collateral for asset-backed securities, and



•

evaluate all bonds and asset-backed securities whose financial performance has declined for other than temporary impairment.

        Publicly-traded preferred stocks.    We review issuer creditworthiness at least quarterly. Creditworthiness factors reviewed include nationally recognized credit rating agency rating changes and changes in financial performance of the underlying issuer. We monitor all preferred stock investments with declining financial performance for other-than-temporary impairment.

        Publicly-traded common stocks.    Quarterly, we review each common stock investment to determine if its decline in value is deemed other-than-temporary. Our review includes an analysis of issuer

36

financial trends, and market expectations based on third-party forward-looking analytical reports, when available.

        Private common and preferred stocks and other invested assets.    We review quarterly private issue valuations, which include recent transaction valuations between the issuer and a third party; financial performance reviews; and financial trend comparisons with publicly-traded companies in the same or similar industries.

        We recognize an other-than-temporary impairment loss when appropriate for these investments with continuous material unrealized losses due to issuer-specific events. This decision is based upon the facts and circumstances for each investment.

Assets with unrealized losses due to market conditions or industry-related events

        Invested assets with unrealized losses due to market conditions or industry-related events include those negatively impacted by increasing U.S. Treasury or local sovereign interest rates; corporate and asset-backed credit spread widening; common stock price volatility due to conditions in the overall market or a particular industry; and illiquid market conditions.

        Under some conditions, it is assumed that a decline in value below cost is temporary. This assumption is made for fixed-maturity investments with unrealized losses due to market conditions or industry-related events when the market is expected to recover, and we have the intent and ability to hold the investment until maturity or the market recovers, which is a decisive factor when considering an impairment loss. If the decision that holding the investment is no longer appropriate, we will reevaluate that investment for other-than-temporary impairment.

        An evaluation is made for other-than-temporary impairment for preferred and common stock and other investments with continuous material unrealized losses for two consecutive quarters due to market conditions or industry-related events. An other-than-temporary impairment loss is recognized based upon each investment's facts and circumstances. We continue to monitor these securities quarterly to ensure that unrealized losses are not the result of issuer-specific events.

Intangible Assets

        Intangible assets represent the excess of cost over the value of net tangible assets of acquired businesses.

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        We:

•

classify our intangible assets as either goodwill, client lists, non-compete agreements, future profits of purchased books of business of the insurance underwriting subsidiaries, or other purchased intangibles;



•

allocate intangible assets between goodwill and other intangible assets and determine estimated useful lives based on our internal valuations or valuations from qualified independent appraisers. We base the calculations of these amounts on estimates and assumptions using historical and pro forma data and recognized valuation methods. Different estimates or assumptions could produce different results;



•

amortize intangible assets other than goodwill over their estimated useful lives, while goodwill is not subject to amortization; and



•

carry intangible assets at cost, less accumulated amortization in the accompanying consolidated statements of financial position.

        Goodwill is not amortized but is tested for impairment at least annually, and more frequently if there are indicators of impairment or whenever business circumstances indicate that the carrying value of goodwill may not be recoverable. Impairment reviews are performed at the reporting unit level. If the fair value of a reporting unit is determined to be less than the carrying value of the reporting unit, we would complete further analysis to determine whether there was an impairment loss. No further analysis was required in 2004 or 2003. We base our determinations of fair value on estimates and assumptions related to the amount and timing of future cash flows and future interest rates. Different estimates or assumptions could produce different results. We included the estimated effects of the regulatory investigations and the termination of contingent commission arrangements in our 2004 impairment test.

        In March 2005 we re-evaluated the results of our annual impairment review due to the subsequent developments on the matters described in Note 14 to our consolidated financial statements and concluded that our initial conclusions remain appropriate and that no impairment loss is required.

REVIEW OF CONSOLIDATED RESULTS

General

        In the discussion of operating results we sometimes refer to supplemental information extracted from consolidated financial information, which U.S. GAAP does not require to be presented in the financial statements.

        Supplemental information related to organic revenue growth is information that helps Aon and our investors evaluate business growth from existing operations. Organic revenue growth excludes from reported revenues the impact of foreign exchange, acquisitions, divestitures, transfers between business units, investment income, reimbursable expenses, unusual items, and for the underwriting segment only, an adjustment between written and earned premium.

        Supplemental organic revenue growth information does not affect net income or any other GAAP reported figures. It should be viewed in addition to, not instead of, our consolidated statements of income. Industry peers provide similar supplemental information about their revenue performance, although they do not make identical adjustments.

        Since Aon conducts business in more than 120 countries and sovereignties, the movements of foreign exchange rates are important to our business. In comparison to the U.S. dollar, foreign exchange rate movements may be significant and may distort true period-to-period comparisons of changes in revenue or pretax income. Therefore, management has isolated the impact of the change in

38

currencies between periods by providing percentage changes on a comparable currency basis for revenue, and has disclosed the effect on earnings per share. This form of reporting is intended to give financial statement users more meaningful information about our operations.

        Some tables in the segment discussions reconcile organic revenue growth percentages to the reported revenue growth percentages for the segments and sub-segments. We separately disclose the impact of foreign currency as well as the impact from acquisitions, divestitures, and transfers of business units, which represent the most significant reconciling items. In an "all other" category, we total other reconciling items that are not generally significant individually or in the aggregate. If there is a significant individual reconciling item within the "all other" category, we provide additional disclosure in a footnote.

Summary Results for 2002 through 2004

        The consolidated results of continuing operations follow:

Consolidated Results for 2004 Compared to 2003

Revenue

        In 2004, revenue increased $454 million or 5% over 2003 to $10.2 billion. The movement in foreign exchange rates caused a majority of this increase, as revenue increased $67 million excluding foreign exchange effects. We do not directly hedge revenues against foreign currency translation because it is not cost effective, but we do try to mitigate the effect of foreign currency fluctuations on pretax income through other hedging strategies.

        Brokerage commissions and fees increased by $263 million or 4% from the prior year, driven almost entirely by favorable foreign exchange rates. There was no organic revenue growth in the Risk and Insurance Brokerage Services segment in large part due to the termination of contingent commission arrangements, as well as a softer insurance market. Consulting organic revenue grew 1%.

        Premiums and other increased $179 million or 7% from the prior year due to increased retentions, a change in an insurance program for a specialty accident and health line, and favorable foreign

39

exchange rates. Growth in specialty property and casualty and core accident, health, and life business was offset by a planned decrease in the runoff businesses.

        Investment income increased by 4% over 2003, and includes related investment expenses and income or loss on investment disposals and impairments. The net increase reflects improved results at the operating segments driven primarily by an increase in short-term rates, partially offset by a decline at Corporate.

        Consolidated revenue by geographic area follows:

        U.S. consolidated revenue, which represents 52% of total revenue, increased $50 million or 1% in 2004 compared to 2003. The low revenue growth reflects the softer U.S. retail market that began late in 2003 after a two-year rapid increase in premiums following the September 11 tragedy and lower contingent commission revenue. Additionally, the November 2004 sale of Cambridge resulted in a $19 million loss of revenue in 2004 as compared to 2003.

        U.K. revenue decreased $24 million or 1%. Excluding the positive effects of foreign currency exchange, revenue decreased $198 million or 10%. The decrease in revenue is attributable to the soft market, which resulted in lower premiums and commissions. Additionally, the sale of our U.K. claims service business resulted in an $82 million loss of revenue in 2004 as compared to 2003.

        Continent of Europe revenue increased $250 million or 17% and Rest of World revenue increased $178 million or 14%, principally reflecting a weakening of the U.S. dollar.

Expenses

        Total expenses increased $705 million or 8% over 2003.

        General expenses increased $393 million or 6% over 2003, reflecting the impact of:

•

foreign exchange rates,



•

increase in our net periodic pension expenses of $47 million for our major plans over 2003, and



•

a $40 million provision for costs and payments to settle the Daniel class action lawsuit.

        Net gains on currency derivative transactions reduced expenses by $45 million in 2004.

        The 6% increase in benefits to policyholders was driven by the combination of growth in underwriting revenue, the change in an insurance program for a specialty accident and health line, and foreign exchange rates. In 2003, expenses increased due to higher claims for National Program Services, Inc. (NPS) of $79 million (see the Consolidated Results for 2003 Compared to 2002 section below for more information).

        Interest expense increased $35 million or 35% primarily due to the adoption of FIN 46 on December 31, 2003, which required the deconsolidation of our trust preferred capital securities, and which was offset by an increase in notes payable. Interest expense on the notes payable was $58 million for 2004. Absent this item, interest expense declined $23 million due principally to a reduction in debt

40

levels during most of the year (see Notes 1 and 10 to the consolidated financial statements for more information).

        Included in our results is a $180 million provision for settlements resulting from investigations by the New York AG and other regulatory authorities.

        In 2003, total expenses included a $14 million credit related to the World Trade Center property insurance settlement. The 2003 credit represents a $60 million final settlement of our World Trade Center property insurance claim, net of $46 million paid to a third party relating to temporary office space secured in Manhattan after the World Trade Center was destroyed.

Income from Continuing Operations Before Income Tax and Minority Interest

        Because the increase in expenses exceeded the increase in revenues (for the reasons described above), income from continuing operations before income tax and minority interest decreased $251 million in 2004 to $880 million.

        Approximately 87% of Aon's 2004 consolidated income from continuing operations before income tax and minority interest was from international operations. The $220 million provisions for settlements resulting from investigations by the New York AG and other regulatory authorities and for costs and payments to settle the Daniel class action lawsuit were considered domestic expenses.

Income Taxes

        The effective tax rate was 34.4% and 37% in 2004 and 2003, respectively. Differences between the overall effective tax rate and the U.S. federal statutory rate are typically due to U.S. state income taxes and differentials between U.S. and international tax rates. Changes in the mix between our U.S. and international pretax income directly impact our effective tax rates. In 2004, a one-time tax benefit resulting from the difference between our tax and book basis in Cambridge reduced our effective tax rate. A summary of these effects is included in the rate reconciliation provided in Note 8 to the consolidated financial statements.

Income from Continuing Operations

        Income from continuing operations decreased to $577 million ($1.72 per diluted share) from $676 million ($2.04 per diluted share) in 2003. Basic income per share from continuing operations was $1.80 and $2.12 for 2004 and 2003, respectively. Including the effect of currency hedges, the positive impact of foreign currency translation was approximately $0.18 per share.

        To compute income per share, we have deducted dividends paid on the redeemable preferred stock from net income. In accordance with Emerging Income Task Force (EITF) No. 04-8, The Effect of Contingently Convertible Investments on Diluted Earnings Per Share, diluted shares outstanding were increased by 14 million to reflect the possible conversion of Aon's 3.5% convertible debt securities. After-tax interest expense on these debt securities has been added back to income from continuing operations when calculating the diluted income per share.

Discontinued Operations

        After-tax losses from discontinued operations in 2004 were $31 million ($0.10 and $0.09 per basic and diluted share, respectively). In comparison, after-tax losses in 2003 from discontinued operations were $48 million ($0.15 and $0.14 per basic and diluted share, respectively).

        Discontinued operations also include:

•

certain insurance underwriting subsidiaries acquired with Alexander and Alexander Services, Inc. (A&A), where we are paying off these subsidiaries' liabilities over several years, and

41

•

payments related to A&A's indemnification of liabilities relating to subsidiaries sold by A&A before its acquisition by Aon.

        There was no material impact on the income statement from the A&A discontinued operations in 2004, 2003 or 2002.

        Based on current estimates, management believes that these A&A discontinued operations are adequately reserved. The net liability is included as a component of other liabilities in the consolidated statements of financial position (see Note 5 to the consolidated financial statements for more information on discontinued operations).

Consolidated Results for Fourth Quarter 2004 Compared to Fourth Quarter 2003

        Total revenue in the quarter rose 3% to $2.7 billion. Excluding the impact of changes in foreign exchange rates, revenue was even with last year. Organic growth declined 1% reflecting:

•

a softening insurance market,



•

lower contingent commission revenue of $37 million, and



•

$19 million in lower revenue from the sale of our U.S. Claims service business effective November 30, 2004.

        The sale of our U.S. and U.K. claims services businesses more than offset the positive impact of acquisitions made by our brokerage operations. Lower claims services revenue (in continuing operations) is primarily due to the sale of Cambridge.

        These shortfalls were partially offset by growth in international risk and insurance brokerage operations.

        Income from continuing operations before income taxes and minority interest decreased by $248 million or 69% from 2003. The significant decrease was influenced by:

•

a provision of $180 million for settlements resulting from the investigation by the New York AG and other regulatory authorities,



•

a $40 million provision for costs and payments to settle the Daniel class action lawsuit and



•

a credit in 2003 of $60 million, representing a final settlement for our overall World Trade Center property insurance claim.

        These were partially offset by:

•

a gain in 2004 of $37 million from the sale of virtually all of our remaining common stock investment in Endurance and



•

a charge in 2003 in the warranty, credit, and property and casualty sub-segment of $45 million for additional losses and reserve strengthening for the NPS run-off program (see description of NPS in the following section under expenses).

Consolidated Results for 2003 Compared to 2002

Revenue

        Total revenues were $9.7 billion, an increase of 11%. Excluding the effects of foreign exchange rates, revenues increased 6% over the comparable period. This increase resulted from improvements in brokerage commissions and fees, premiums earned, and investment income.

42

        Brokerage commissions and fees increased 11% to $6.8 billion as a result of:

•

growth in new business,



•

improved client retention rates for most of our businesses,



•

the weakening U.S. dollar, and



•

higher revenue from a large Consulting segment outsourcing contract begun in the third quarter 2002.

        Premiums and other, primarily related to insurance underwriting operations, improved to $2.6 billion, a 10% increase over 2002. The increase reflects growth in some warranty and credit programs, along with specialty property and casualty lines, and favorable foreign exchange rates.

        Investment income increased 24% over 2002, and includes related investment expenses and income or loss on investment disposals and impairments. The net increase reflects:

•

lower impairment write-downs in 2003 of $36 million compared with $130 million in 2002. 2002 impairment write-downs included a $51 million cumulative adjustment related to prior reporting periods,



•

a non-cash increase in the value of Endurance stock warrants of $80 million and equity earnings from our investment in Endurance common stock of $25 million. Equity earnings from our Endurance investment in 2003 and 2002 were $46 million and $21 million, respectively,



•

interest income in 2002 on a tax-related settlement of $48 million with no corresponding amount in 2003, and



•

lower investment income generated by the operating units of $85 million including $27 million on deposit-type contracts which reflected lower rates.

        U.S. revenues, which represent 54% of total revenue, increased 4% in 2003 compared to 2002 as a result of growth driven:

•

primarily by new business development and improved retention rates in both retail and reinsurance brokerage and



•

secondarily by growth in the Consulting segment resulting from a large outsourcing contract that began in the third quarter 2002. This growth was partially offset by a decrease in revenue in our Accident & Health and Life sub-segment that was primarily from transferring our U.S. large employer group life and accidental death business to a third party via an indemnity reinsurance arrangement.

        While moderating, commercial property and casualty premium rate increases for most lines of coverage continued in 2003. We generally benefit from increased premium rates through increased commissions, although higher premiums can cause clients to purchase lower policy limits and retain higher deductibles.

        Combined U.K. and Continent of Europe revenues increased 21% in 2003 to $3.2 billion and Rest of World revenue increased 23% to $1.3 billion, reflecting foreign exchange, strong new business, and the positive effect of increased premium rates.

Expenses

        General expenses increased 10% over 2002 reflecting:

•

growth of the businesses,



•

the effect of foreign exchange rates, and

43

•

higher overall defined benefit pension plan costs of $131 million for our major plans.

        General expenses in 2002 included $50 million of costs from the planned divestiture of the insurance underwriting segment and a credit of $6 million, reflecting the reversal of termination benefits that we incurred as part of our business transformation plan.

        Benefits to policyholders rose $52 million, or 4%, primarily as the result of new business volume, and losses and reserve strengthening of $65 million relating to NPS business, a non-core book of runoff business. NPS was hired to handle quoting, binding, premium collection, claims adjusting, and other servicing related to general liability insurance policies issued by one of Aon's subsidiaries. However, in mid-2002, we obtained a temporary restraining order against NPS, stopped it from initiating any new business on our behalf, and with others, sued the company for fraud. In 2003, actuaries examined the business that NPS had written and reviewed assumptions, such as historical loss development patterns and the expected ultimate loss ratio. As a result of this review, we strengthened our reserves, mainly for accident years 2001 and 2002. These reserves should cover future payments we expect to make in the next five to seven years. Excluding NPS, benefit payout ratios have declined, however, due to a shift in product mix.

        Interest expense decreased primarily due to lower debt levels. Amortization of intangible assets grew $9 million from 2002 due primarily to recent acquisitions as well as foreign exchange rates in the risk and insurance brokerage services segment.

        Total expenses also included a $14 million credit in 2003 and a $29 million credit in 2002 related to the World Trade Center. The 2003 credit represents a $60 million gain from a final settlement of our World Trade Center property insurance claim, net of $46 million related to the assignment to a third party of temporary office space we needed in Manhattan after the World Trade Center was destroyed. The 2002 credit represents a gain resulting from a settlement with our insurance carriers regarding reimbursement for depreciable assets that were destroyed.

Income from Continuing Operations Before Income Tax and Minority Interest

        Income from continuing operations before income tax and minority interest increased from $840 million in 2002 to $1.1 billion in 2003. This increase is due primarily to the impact of foreign exchange, the improvement in investment income ($61 million), and 2002 expenses related to the planned spin-off ($50 million) with no corresponding amount in 2003. Approximately 68% of Aon's 2003 consolidated income from continuing operations before income tax and minority interest was from international operations.

Income Taxes

        The effective tax rate was 37% in both 2003 and 2002. The overall effective tax rates are higher than the U.S. federal statutory rate primarily because of state income tax provisions.

Income from Continuing Operations

        Income from continuing operations increased to $676 million ($2.04 per diluted share) from $496 million ($1.75 per diluted share) in 2002. Basic income per share from continuing operations was $2.12 and $1.76 for 2003 and 2002, respectively. In fourth quarter 2002, we had a common stock offering, which increased the number of average common and common stock equivalent shares outstanding. Including the effect of currency hedges, the positive impact of foreign currency translations was approximately $0.12 per share. We have deducted dividends paid for the redeemable preferred stock from net income to compute income per share. In accordance with EITF No. 04-8, 2003 diluted shares outstanding were increased by 14 million to reflect the possible conversion of Aon's 3.5%

44

convertible debt securities. After-tax interest expense on these debt securities has been added back to income from continuing operations when calculating the diluted income per share.

Discontinued Operations

        After-tax losses from our discontinued businesses in 2003 were $48 million ($0.15 and $0.14 per basic and diluted share, respectively). In comparison, losses in 2002 from these discontinued operations were $30 million ($0.11 per both basic and diluted share). The 2003 results include an after-tax loss on the revaluation of the automotive finance business of $14 million.

REVIEW BY SEGMENT

General

        Aon classifies its businesses into three operating segments: Risk and Insurance Brokerage Services, Consulting, and Insurance Underwriting (see Note 15 to the consolidated financial statements for further information). Aon's operating segments are identified as those that:

•

report separate financial information and



•

are evaluated regularly when we are deciding how to allocate resources and assess performance.

        We attribute revenues to geographic areas based on the location of the resources producing the revenues.

        Segment revenue includes investment income generated by invested assets of that segment, as well as the impact of related derivatives. Investment characteristics mirror liability characteristics of the respective segments:

•

Our Risk and Insurance Brokerage Services and Consulting businesses invest funds held on behalf of clients and operating funds in short-term obligations.



•

In Insurance Underwriting, policyholder claims and other types of non-interest sensitive insurance liabilities are primarily supported by intermediate to long-term fixed-maturity instruments. Investments underlying interest-sensitive capital accumulation insurance liabilities are fixed- or floating-rate fixed-maturity obligations. For this business segment, operating invested assets are approximately equal to average net policy liabilities.



•

Our insurance subsidiaries also have invested assets that exceed net policy liabilities which allow us to maintain solid claims paying ratings. Income from these investments are reflected in Corporate and Other segment revenues.

45

        The following tables and commentary provide selected financial information on the operating segments.

(1)

Intersegment revenues of $72 million and $68 million were included in 2004 and 2003, respectively. See Note 15 to the consolidated financial statements for further information.

Risk and Insurance Brokerage Services

        Aon is a leader in many sectors of the insurance industry: globally, it is the second largest insurance broker, the largest reinsurance broker and the leading manager of captive insurance companies worldwide. In the U.S., Aon is the largest wholesale broker. These rankings are based on the most recent surveys compiled and reports printed by Business Insurance.

        Changes in premiums have a direct and potentially material impact on the insurance brokerage industry, as commission revenues are generally based on a percentage of the premiums paid by insureds. More specifically, lower premium rates, or a "soft market," generally result in decreased commission revenues.

        After the attacks of September 11, 2001, premium rates saw an unprecedented increase. Since late 2003, however, premiums in the property and casualty marketplace have declined. The downward rate trend varies by line of business, area of the world, and when each line of business began its downward trend. This trend may inhibit brokers' ability to grow revenues.

        Risk and Insurance Brokerage Services generated approximately 57% of Aon's total operating segment revenues in 2004. Revenues are generated primarily through:

•

commissions and fees paid by insurance and reinsurance companies,



•

fees paid by clients,



•

other carrier compensation, and



•

interest income on funds held on behalf of clients.

        Our revenues vary from quarter to quarter throughout the year as a result of:

•

how our clients' policy renewals are timed,



•

the net effect of new and lost business,



•

the timing of services provided to our clients, and



•

the income we earn on investments, which is heavily influenced by short-term interest rates.

46

        Our retail brokerage companies operate in a highly competitive industry and compete with many retail insurance brokerage and agency firms, as well as individual brokers and agents and direct writers of insurance coverage. Specifically, this segment:

•

addresses the highly specialized product development and risk management needs of commercial enterprises, professional groups, insurance companies, governments, healthcare providers, and non-profit groups, among others;



•

provides affinity products for professional liability, life, disability income and personal lines for individuals, associations and businesses;



•

provides wholesale brokerage, managing underwriting and premium finance services to independent agents and brokers as well as corporate clients;



•

provides actuarial, loss prevention and administrative services to businesses and consumers; and



•

offers claims management and loss cost management services to insurance companies and firms with self-insurance programs. During 2004, we exited most of these activities by completing the sale of our U.K. claims operations in the second quarter of 2004 and our U.S. third party claims administration business in fourth quarter 2004.

        We review our revenue results using the following sub-segments:

•

Risk Management and Insurance Brokerage — Americas (Brokerage — Americas) encompasses our retail and wholesale brokerage services, affinity products, managing general underwriting, placement and captive management services, and premium finance services in North and South America, the Caribbean and Bermuda.



•

Risk Management and Insurance Brokerage — International (Brokerage — International) offers similar products and services to the parts of the world not included in Brokerage-Americas.



•

Reinsurance Brokerage and Related Services (Reinsurance) offers sophisticated advisory services in program design and claim recoveries that:



—

enhance the risk/return characteristics of insurance policy portfolios,



—

improve capital utilization, and



—

evaluate and mitigate catastrophic loss exposures worldwide.



•

Claims Services (Claims) offered claims administration and loss cost management services. We exited most of these activities in 2004 by selling our U.S. and U.K. claims administration businesses.

Revenue

        Continuing the trend from late 2003, the insurance market softened further in 2004. Total 2004 Risk and Insurance Brokerage Services revenue was $5.7 billion or 3% over last year. Excluding the effect of foreign exchange rates, revenue declined 1% over last year as a result of:

•

soft market conditions, reflected in flat organic revenue,



•

a $101 million reduction in revenue from our U.S. and one of our U.K. claims services businesses which were sold during 2004, and



•

a $35 million decline from contingent commissions.

47

        This table details Risk and Insurance Brokerage Services revenue by sub-segment.

        This table reconciles organic revenue growth to reported revenue growth in 2004 versus 2003.

•

Brokerage—Americas revenue was up 1%, reflecting the impact of favorable foreign exchange rates. Organic revenue growth was zero, reflecting pricing pressures resulting from an overall soft market and lower contingent commission revenue, which was offset by improved retention rates.



•

Brokerage—International revenue showed strong improvement as a result of a favorable foreign exchange impact and good organic growth, especially in Asia, France and Italy.



•

Reinsurance revenue decreased because new business growth was more than offset by lower pricing on retained clients, lower renewal business in the Americas, and higher risk retention by clients.



•

Claims revenue was $82 million lower in 2004 compared to 2003 due to the sale of one of our U.K. claims services businesses in January 2004. In addition, revenue declined due to the sale of our U.S. claims services business in November 2004, and lower levels of activity in our U.S. operation during the year.

        This table shows Risk and Insurance Brokerage Services revenue by geographic area and total pretax income:

•

U.S. revenue decreased 4% over 2003 due to lower contingent commissions and declines in our wholesale, premium financing, underwriting management, and claims service businesses.

48

•

U.K. revenue declined 3%. Excluding the favorable impact of foreign exchange rates, revenue would have declined 11%. Both the retail and reinsurance sectors declined, primarily as a result of decreased pricing pressures, higher retention of risk by clients and lower new business. In addition, revenue declined $82 million due to the sale in the first quarter 2004 of one of our U.K. claims services businesses.



•

Continent of Europe revenue increased 14% driven by a favorable foreign currency impact along with strong organic revenue growth in France and Italy.



•

The 13% growth in the Rest of World is driven by a favorable foreign currency impact and growth in Asia.

Income Before Income Tax

        Pretax income decreased $219 million or 26% from 2003 to $629 million, despite the increase in revenue. In 2004, pretax margins in this segment were 11.0%, down from 15.2% in 2003.

        Our income and margins were affected this year by:

•

a $153 million provision allocation for regulatory settlements resulting from the investigation by the New York AG and other regulatory authorities,



•

a $35 million reduction in contingent commission revenue,



•

additional net periodic pension expense of $39 million,



•

a $34 million provision for costs and payments to settle the Daniel class action lawsuit,



•

premium declines on brokered coverage, and



•

increase in investment income of $13 million.

Consulting

        Aon Consulting is one of the world's largest integrated human capital consulting organizations. This segment:

•

provides a full range of human capital management services from employee benefits to compensation consulting and



•

generated 12% of Aon's total operating segment revenues in 2004.

        We review our revenue results using the following sub-segments:

•

Consulting Services, which provides human capital consulting services in five practice areas:



1.

Employee Benefits advises clients about the structure, funding and administration of employee benefit programs which attract, retain and motivate employees. Benefits consulting includes health and welfare, retirement, executive benefits, absence management, compliance, employer commitment, investment advisory and elective benefit services.



2.

Compensation focuses on designing salary, bonus, commission, stock option and other pay structures, with special expertise in the financial services and technology industries.



3.

Management Consulting assists clients in process improvement and design, leadership, organization and human capital development, and change management.



4.

Communications advises clients on how to communicate initiatives that support their corporate vision.

49

5.

Strategic Human Resource Consulting advises complex global organizations on talent, change and organization effectiveness issues including assessment, selection performance management, succession planning, organization design and related people-management programs.



•

Outsourcing, which offers employment processing, performance improvement, benefits administration and other employment-related services.

Revenue

        In 2004, revenues of $1.2 billion increased 5% over 2003. Revenue on an organic basis grew 1% from last year and resulted from growth in U.S. compensation and international practices, especially in Europe, offset by lower revenue in U.S. employee benefits.

        In connection with our termination of contingent fee arrangements, contingent commissions decreased $2 million from the fourth quarter 2003 compared to the fourth quarter 2004.

        This table details Consulting revenue by sub-segment.