A systematic review of the literature from 1966 to 2002 was performed to determine the rate at which autopsies detect important, clinically missed diagnoses and the extent to which this rate has changed over time. Fifty-three autopsy series were studied that reported diagnoses involving a primary cause of death (major errors) and those likely to have affected patient outcome (class I errors). The median major error rate was 23.5% (4.1% to 49.8%), and the median class I error rate was 9.0%. Over time, there were relative decreases in major errors and class I errors of 19.4% and 33.4% per decade, respectively. Despite these decreases, the authors estimate that for modern U.S. institutions, there is likely a major error rate from 8.4% to 24.4% and a class I error rate from 4.1% to 6.7%.