Pathologic investigation of 50 children, adolescents, and young adults who died suddenly and unexpectedly revealed that the most frequently encountered cardiovascular diseases were mitral valve prolapse (12 cases), myocarditis (12 cases), hypertrophic cardiomyopathy (six cases), and anomalous origin of a coronary artery from the aorta (two cases). Five subjects had no evidence of cardiac abnormalities. A striking family history of sudden death was encountered among eight subjects, of whom three had mitral valve prolapse and three had normal hearts. Assessing the circumstances surrounding sudden death, it was found that at the time of collapse, 32 subjects were engaged in regular activity, eight subjects were engaged in active athletics, and ten were found dead in bed. A relationship of the terminal event to emotional stress was reported in three subjects.