By geometric analysis of real-time 3-dimensional echocardiography performed in 39 patients with hypertrophic obstructive cardiomyopathy and definite systolic anterior motion of the mitral valve, we found that the angle between the mitral annular plane and basal portion of the anterior mitral valve leaflet, and the angle between the basal portion of the anterior leaflet and its tip portion measured in the medial and central anteroposterior planes, were significantly smaller than those in the lateral plane. The distance between the interventricular septum and the anterior mitral valve tip in the medial and central plane was also significantly smaller than that in the lateral plane. The lateral distance between the interventricular septum and the anterior mitral valve tip was the only independent determinant of left ventricular outflow tract pressure gradient by multiple stepwise regression analysis. In conclusion, systolic anterior motion of the mitral valve develops predominantly in the medial side, resulting in laterally located narrow left ventricular outflow tract opening in patients with hypertrophic obstructive cardiomyopathy.