Postmortem examinations performed on 76 children with a clinical diagnosis of congenital heart disease were reviewed retrospectively and compared with the findings before death. Both operated and unoperated cases were studied over a three year period. Despite intensive investigation during life, there was a high rate of unsuspected abnormalities at necropsy (80%): 29 cases had undiagnosed additional cardiac anomalies or surgical flaws, which contributed to death in 13 cases. Defects in surgery were uncommon but permitted modification in surgical technique to avoid recurrence. Myocardial necrosis and pulmonary foreign body embolism were common findings, the importance of which is uncertain and requires further study for their prevention. Even in the most thoroughly investigated cases postmortem examination has a high yield of clinically important pathology which is undetected during life.