Although death during exercise is rare, vigorous physical activity is associated with increased risk for fatality, particularly in individuals with overt cardiac disease or a high coronary risk profile. The mechanism of death is usually a lethal ventricular arrhythmia, but this may vary depending on the underlying cardiac condition. Cardiac disease is present in the great majority of individuals who die during exercise. In young persons, hypertrophic cardiomyopathy and congenital coronary anomalies are most frequent, whereas older victims usually have coronary artery atherosclerosis. Cardiac disease is typically unrecognized before the fatal event in young individuals; in the older group, most have overt coronary disease or identifiable risk factors. Screening asymptomatic subjects to identify increased cardiac risk during exercise is problematical in terms of logistics, expense and accuracy. However, careful evaluation, including exercise testing, is mandatory before a program of increased physical activity is initiated in cardiac patients. For other individuals, firm guidelines are lacking, and the extent of the evaluation must be determined on an individual basis.