The causes of sudden and unexpected death in 29 highly conditioned, competitive athletes, ages 13-30 years, are summarized. Sudden death occurred during or just after severe exertion on the athletic field in 22 of the 29 athletes. Structural cardiovascular abnormalities were identified at necropsy in 28 of the 29 athletes (97%), and in 22 (76%) were almost certainly the cause of death. The most common cause of death in this series was hypertrophic cardiomyopathy, which was present in 14 athletes. Other cardiovascular abnormalities that occurred in more than one athlete were anomalous origin of the left coronary artery from the right (anterior) sinus of Valsalva, idiopathic concentric left ventricular hypertrophy, coronary heart disease and ruptured aorta. Cardiac disease was suspected during life in only seven of the 29 patients, and in only two of the seven was the correct diagnosis made clinically. Hence, in this series of young athletes, sudden death was usually due to structural cardiovascular disease, and hypertrophic cardiomyopathy was a frequent cause of sudden death; atherosclerotic coronary heart disease was relatively uncommon.