Running is an excellent means of conditioning the cardiovascular system. It is a highly aerobic activity that utilizes both fatty acids and carbohydrates for energy. The typical runner tends to have a slow resting pulse rate and a high maximal oxygen consumption. Echocardiographic studies show that distance runners have larger, thicker left ventricles than do sedentary controls; their hearts are more efficient than those of sedentary people, pumping a larger volume per beat. Physiologic findings on examination of well-trained runners can sometimes be confused with pathologic entities. The "athlete's heart," once believed to be an abnormal condition, is now recognized as representing a highly efficient organ. Runners are not immune to organic heart disease. Careful physical examination of the cardiovascular system, the resting EKG, and echocardiography are useful ways to screen young competitive athletes for cardiomyopathy, the leading cause of sudden death in this age group. Treadmill testing is often used as a screening test in middle-aged and older runners for underlying coronary atherosclerotic heart disease, the leading cause of sudden exercise-related death in this age category. Radionuclide exercise scans and, on occasion, coronary angiography will be of use when the diagnosis of coronary disease is still in doubt. Distance running has positive effects on a host of coronary risk factors, which may help to explain why chronic endurance exercise is associated with lower coronary death rates.