Studies were made with standard time motion and B-scan echocardiography on 48 patients including 5 with idiopathic hypertrophic subaortic stenosis (hypertrophic obstructive cardiomyopathy), undergoing diagnostic cardiac catheterization. The dimensions of the left ventricular outflow (O) and inflow (I) tracts were measured on the B-scan images. The outflow tract was significantly narrowed in idiopathic hypertrophic subaortic stenosis at both end-systole (1-1+/-0-1 cm) and end-diastole (1-3+/-0-1 cm) when compared with the average width in other patients (2-6+/-0-1 and 3-0+/-0-1 cm, at end-systole and end-diastole, respectively) (P less than 0-001) or normal subjects (2-4+/-0-3 and 2-9+/-0-2 cm) (P less than 0-01). Furthermore, the O/I ratio differed significantly in idiopathic hypertrophic subaortic stenosis (0-5+/-0-1 at end-systole and 0-6+/-0-1 at end-diastole) from that in all other groups (1-4+/-0-1 at both end-systole and end-diastole) (P less than 0-005). There was no appreciable change in the width of the outflow tract from mid- to end-systole in the two patients in whom this was examined. The data support the contention that the anterior leaflet of the mitral valve assumes an abnormally anterior position in idiopathic hypertrophic subaortic stenosis. Though the systolic anterior movement of the tip of the anterior leaflet of the mitral valve shown by M-mode echocardiography could not readily be confirmed with B-scans, we believe that the narrowed outflow tract found in the present investigation contributes to the obstruction that occurs in this disease. We suggest that this outflow tract narrowing is probably caused by hypertrophy of the ventricular septum which in itself contributes to the narrowing, but which also displaces the papillary muscles and thus produces abnormal traction on the mitral valve and striking anterior displacement of the valve apparatus.