In order to investigate sudden cardiac death at exertion (sports, and light work in daily life), 226 cases of sudden death during sports and 1,729 autopsy cases of sudden death were studied, and exercise tests were performed on 138 patients with various cardiac diseases. Among sports, marathon or jogging caused sudden death most frequently (52.7%). It occurred often immediately after goal (41.3%) or before goal (28.0%), but cases with a history of heart disease died at the beginning of the race. Severe heat, overwork and other factors may precipitate the sudden death. In autopsy cases of sudden cardiac death which occurred at exertion, coronary arteriosclerosis (C) was found in 67.0%, cardiac hypertrophy of unknown etiology (H) in 7.8% and valvular heart diseases (V) in 7.1%. Sudden death in patients with C or H were more frequent at exertion than at rest. Patients with C were older and died at light work. Those with H of a high degree also died at light work, but the younger ones with a mild H died at sports activity. At exercise test, in patients with arrhythmia or hypertrophic cardiomyopathy, abnormalities may occur after exercise. Sudden death at exertion may depend on the predisposing heart disease. Care should be taken especially at the end of the race.