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GAO has prepared this report under the Comptroller General's authority as part of a continued effort to assist Congress in reviewing concentration in the market for public company audits. The small number of large international accounting firms performing audits of almost all large public companies raises interest in potential effects on competition and the choices available to large companies needing an auditor. This report examines (1) concentration in the market for public company audits, (2) the potential for smaller accounting firms' growth to ease market concentration, and (3) proposals that have been offered by others for easing concentration and the barriers facing smaller firms in expanding their market shares. GAO surveyed a random sample of almost 600 large, medium, and small public companies on their experiences with their auditors. GAO also interviewed the four largest accounting firms and surveyed all other U.S. accounting firms that audit at least one public company. GAO also developed an econometric model that analyzed the extent to which various factors, including concentration and new auditing requirements, affected fee levels. To supplement this work, GAO interviewed market participants, including public companies, investors, accounting firms, academics, and regulators. This report makes no recommendations.

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