To evaluate the prevalence and importance of "physiological" left ventricular hypertrophy (LVH) in athletes, 30 marathon runners were studied. Electrocardiographic evidence of LVH, present in 24 subjects, was associated with concomitant echocardiographic evidence of a thickened left ventricular (LV) posterior wall in five, with an enlarged end-diastolic LV diameter in three, as well as with roentgenographic evidence of cardiomegaly in three subjects. Echocardiographic evidence of superior LV function was noted in six. Maximal treadmill stress provoked no ischemic change. Cardiac hypertrophy and dilatation commonly develop in well-conditioned athletes, but the changes in LV size are slight. This "physiological" type of LVH does not lead to an ischemic response during exercise, in contrast to that which is seen in pathological forms of LVH. Physiological LVH may, however, contribute to superior ventricular function.