Sudden death during exercise is rare, but limited data suggest that vigorous physical activity is associated with an increased risk for this event, particularly in individuals with overt cardiac disease or a high coronary risk profile. The mechanism of exercise-associated sudden death is usually a lethal arrhythmia; however, this may vary depending on the underlying cardiovascular disease. In the great majority of cases, cardiovascular disease is present in persons who die during exercise. In young individuals (less than 35 years old) hypertrophic cardiomyopathy and congenital coronary anomalies are the most common conditions, whereas older victims usually have coronary artery disease. Cardiac disease is typically unrecognized prior to death in young persons; in the older population, most have overt coronary disease or recognizable coronary risk factors. Screening asymptomatic individuals to identify increased risk of a cardiac event during exercise presents major problems in terms of logistics, expense, and accuracy, but careful evaluation, including exercise testing, is mandatory before a program of increased activity in patients with overt cardiac disease. In other cases, the extent of any evaluation must be determined on an individual basis.