The ECG and Frank VCG were compared to the hemodynamic findings in 33 patients with idiopathic hypertrophic subaortic stenosis in whom cardiac catheterization had excluded concomitant valvular heart disease, congenital heart disease, or occlusive coronary artery disease. The patients were divided into two groups according to the absence (Group I) or presence (Group II) of left ventricular hypertrophy on the ECG and/or VCG. The 11 patients in Group I were found to have neither mitral insufficiency nor a resting left intraventricular gradient, and only six patients in whom mitral valve movement was visualized demonstrated systolic anterior movement of the anterior leaflet. The papillary muscles and left ventricular wall were either normal or only mildly hypertrophied in 10 of 11 Group I patients. Group II (22 patients) demonstrated either a resting left intraventricular gradient and/or mitral insufficiency in 18 patients. Twenty-one of the 22 patients showed systolic anterior movement of the anterior leaflet of the mitral valve on a cineangiogram and the papillary muscles and left ventricular wall were moderately to severely hypertrophied in 18 patients. These data suggest that specific hemodynamic and anatomic characteristics of hypertrophic subaortic stenosis may be predicted with reasonable accuracy from the ECG and VCG.