The pathogenesis of angina pectoris in patients with aortic stenosis and normal coronary arteries remains uncertain. Using a specially designed Doppler probe, we measured the maximal velocity of coronary blood flow in the left-anterior descending coronary artery at the time of elective open-heart surgery in 14 patients with aortic stenosis and left ventricular hypertrophy (13 had angina) and in 8 controls without left ventricular hypertrophy. The ratio peak velocity of coronary blood flow, after a 20-second occlusion, to resting velocity was decreased by more than 50 per cent (P less than 0.05) in the patients with aortic stenosis. In 7 of the patients this ratio was decreased by more than 75 per cent. Studies of the velocity of coronary blood flow in vessels perfusing the right ventricle in these patients showed only mild abnormalities. These data demonstrate a selective and marked decrease in coronary reserve to the hypertrophied left ventricle in patients with severe aortic stenosis. The impairment in coronary reserve is probably an important contributor to the pathogenesis of angina pectoris in these patients.