Six highly trained marathon runners developed myocardial infarction. One of the two cases of clinically diagnosed myocardial infarction was fatal, and there were four cases of angiographically-proven infarction. Two athletes had significant arterial disease of two major coronary arteries, a third had stenosis of the anterior descending and the fourth of the right coronary artery. All these athletes had warning symptoms. Three of them completed marathon races despite symptoms, one athlete running more than 20 miles after the onset of exertional discomfort to complete the 56 mile Comrades Marathon. In spite of developing chest pain, another athlete who died had continued training for three weeks, including a 40 mile run. Two other athletes also continued to train with chest pain. We conclude that the marathon runners studied were not immune to coronary heart disease, nor to coronary atherosclerosis and that high levels of physical fitness did not guarantee the absence of significant cardiovascular disease. In addition, the relationship of exercise and myocardial infarction was complex because two athletes developed myocardial infarction during marathon running in the absence of complete coronary artery occlusion. We stress that marathon runners, like other sportsmen, should be warned of the serious significance of the development of exertional symptoms. Our conclusions do not reflect on the possible value of exercise in the prevention of coronary heart disease. Rather we refute exaggerated claims that marathon running provides complete immunity from coronary heart disease.