To determine the extent of agreement on underlying cause of death between death certificates and autopsy reports, we analyzed 272 randomly selected autopsy reports and corresponding death certificates from among all such data on autopsies performed in Connecticut in 1980. In 29 per cent of the deaths, a major disagreement on the underlying cause of death led to reclassification of the death in a different International Classification of Diseases major disease category. In an additional 26 per cent, the death certificate and autopsy report agreed on the major disease category but attributed the death to a different specific disease. Deaths due to neoplasms were most accurately diagnosed, with a sensitivity of 87 per cent and a positive predictive value of 85 per cent. Deaths resulting from diseases of the respiratory or digestive system were associated with the highest rates of disagreement. Diseases most commonly overdiagnosed were circulatory disorders, ill-defined conditions, and respiratory diseases. Diseases most commonly underdiagnosed as the cause of death on the death certificate were specific traumatic conditions and gastrointestinal disorders. The autopsy remains an important method for ensuring the quality of mortality statistics.