A 44-year-old trained marathon runner collapsed after completing 24 miles of the 1973 Boston Marathon. He was resuscitated from ventricular fibrillation. Death occurred after 50 days of coma. Extensive transmural anterior myocardial infarction was documented on electrocardiogram and proved at autopsy, yet the coronary arteries were free of significant atherosclerosis. We believe this report to be the first documentation of a myocardial infarction in a trained athlete while participating in a marathon. We emphasize that the relation between exertion and infarction is unknown. Advocates of long distance running for prevention of, or rehabilitation from, ischemic heart disease should be aware of this possible complication.