Background: We sought to investigate the relationship between geometric changes of mitral apparatus and degree of trans-left ventricular outflow tract (LVOT) pressure gradient (PG) in patients with hypertrophic cardiomyopathy using three-dimensional (3D) echocardiography. Methods: Utilizing real-time 3D echocardiography, we obtained transthoracic volumetric images in 17 patients with HCM presenting with different severity of LVOT pressure gradient and in 5 controls. Original software was used to crop the 3D data into 18 radial planes, and we marked the mitral annulus and leaflets in each plane in mid-systole and mid-diastole. The 3D images of the leaflets and annulus were reconstructed for the quantitative measurements. Also, we obtained LVOT pressure gradient and degree of MR by two-dimensional echocardiography. Results: In obstructive HCM, the annulus flattened with apparent tenting of the leaflets.(6.87짹3.18mm/m2 vs 9.33짹2.07mm/m2 in annular height/BSA; 3.34짹1.51ml/m2 vs 1.67짹1.23ml/m2 in midsystolic tenting volume/BSA). In subgroup analysis according to LVOT PG, patients with higher LVOT PG(n= 8) has larger tenting indices compared with lower LVOT PG (n= 9). However LV ejection fraction and maximal thickness of intervebtricular septum didn���t differ between two groups. Conslusions: Obstructive HCM is associated with apparent mitral annular flattening and tenting of the leaflets. Furthermore, degree of mitral leaflet tenting is significantly associated with severity of LVOT PG.