GAO reviewed the results of the Internal Revenue Service's (IRS) efforts to audit the tax returns of about 45,000 large corporations, focusing on: (1) audit trends for fiscal years 1988 through 1994; (2) the portion of taxes recommended by agents that were eventually assessed; and (3) the profiles of audited large corporations compared with those of nonaudited corporations.

GAO found that: (1) for every dollar invested in large corporation audits, IRS ultimately assessed $15 in additional taxes for the years 1988 through 1994; (2) IRS invested more hours in directly auditing large corporations but recommended less additional tax per hour invested in 1994 compared to 1988; (3) in 1994, large corporations appealed 66 percent of the additional taxes that IRS recommended in its audits; (4) between 1988 and 1994, IRS assessed 27 percent of the recommended additional taxes either after agreement or resolution in appeals; (5) IRS believed that the assessment rate was not an accurate measure of audit effectiveness, since various factors outside the audit could lower the rate; (6) the assessment rates ranged from 20 to 38 percent for four asset classes and from 0 to 103 percent by IRS district, but the reasons for the disparities were unclear; (7) the rates for audits closing without any adjustments has rapidly increased, raising questions about how IRS selects returns for audits; and (8) audited corporations tended to report higher average incomes, tax liabilities, and other tax amounts than nonaudited corporations.