One hundred eighteen patients with idiopathic hypertrophic subaortic stenosis were studied with cardiac catheterization and coronary arteriography. In 112 a gradient across the left ventricular outflow tract was present in the resting state. Seventeen of the 61 patients who had right heart catheterization had a mild resting gradient across the right ventricular outflow tract, that was considered clinically and hemodynamically insignificant. Ninety-five patients (80 percent) had a left ventricular end-diastolic pressure greater than 10 mm Hg; 60 percent had mitral regurgitation that was of mild degree in most cases. Almost 20 percent had coexistent coronary atherosclerosis (25 percent incidence rate in subjects aged 40 years or older). Patients with associated severe coronary atherosclerosis had a lower intraventricular gradient at rest than other patients. Coronary atherosclerosis appears to be a coincidental condition. The need for objective evaluation of the coronary circulation is emphasized.