Backgrounds:We hypothesized that geometric differences in mitral valve leaflets and subvalvular apparatus could determine development of systolic anterior motion of mitral valve (SAM) and subsequent LV outflow tract obstruction (LVOTO) in patients with asymmetric septal hypertrophy (ASH). Methods : Real time 3D echocardiography was done in 33 patients with ASH and 25 controls. Patients included 15 with resting LVOTO (Group I) and 18 without resting or provoked LVOTO (Group II). A customized software (Omni4D) was used to measure anterior and posterior mitral leaflet areas (MLA) at end diastole and other geometric indices including annular area, annular height, septal wall thickness, distances and angles between papillary muscles (PM), fibrous trigone, and mitral centroid. Leaflet areas and all distances were indexed to body surface area (BSA) Results:Septal wall thickness was not different in patient groups. Anterior MLA was larger in both patient groups compared to control, which was significantly larger in Group I than in Group II (9.09짹2.90, 7.22짹2.90 vs. 5.04짹1.23 cm2, respectively, p<0.001).Posterior MLA was also larger in patient groups than in controls, but there were no differences between groups. Mitral annular area and height were not different between patients and controls. The distances or angles among medial PM and other anatomical landmarks were not different among three groups. However, the distance between lateral PM and medial trigone was significantly shorter in Group I, resulting in shorter interpapillary muscle distance. Multivariate logistic regression showed that anterior MLA was the only independent predictor of LVOTO in ASH patients (odds ratio = 2.3, p<0.001) Conclusion:Larger MLA is a characteristic finding in ASH patients and anterior MLA is associated with development of LVOTO.