The spectrum of coronary arterial involvement is described in 42 patients with hemodynamically proved idiopathic hypertrophic subaortic stenosis. Of nine patients with significant coronary arterial involvement, six had atheromatous disease and three had coronary arterial anomalies. The hemodynamic and clinical features of patients with coronary arterial involvement did not differ significantly from those of patients with normal coronary arteries. Neither the severity nor the character of the chest pain distinguished those patients with coronary disease. The diagnostic electrocardiographic features of myocardial infarction classically seen in patients with idiopathic hypertrophic subaortic stenosis were not present in the 9 patients with coronary arterial involvement but were found in 8 of the 33 patients with normal coronary arteries. The data demonstrate a significant incidence of coronary arterial involvement in patients with idiopathic hypertrophic subaortic stenosis after age 45 years.