Echocardiographic study was performed on 21 cases with hypertrophic cardiomyopathy (HCM), 10 obstructive and 11 nonobstructive. Asymmetric septal hypertrophy was demonstrated in both obstructive and nonobstructive HCM. In all cases of HCM studied, the thickness of the interventricular septum was 1.4 cm or more (1.4-3.7 cm) and the ratio of septal to left ventricular posterior wall thickness was 1.4 or more (1.4-3.2). A systolic anterior movement of the mitral valve (SAM) was observed in obstructive cases only and characterized by a large backward component in late systole and an extreme approximation to the interventricular septum at its peak. Patients with HCM also showed abnormal echocardiographic indices of the left ventricular diastolic properties, such as the rate of diastolic descent of the mitral valve (DDR), mean diastolic posterior wall velocity (mPWVd), D/S ratio and mean rapid filling rate (mRFR). DDR correlated well with the ratio of rapid filling to total filling volume, rapid filling fraction, (r = 0.79, p less than 0.001), suggesting that reduced DDR in HCM was related with an abnormal left atrio-ventricular flow pattern. A significant correlation was also observed between mRFR and negative maximum dp/dt derived from the first derivative curve of the left ventricular pressure (r = o.68, p less than 0.005). Thus echocardiography was proved to be a valuable means for investigation of the left ventricular properties during early diastole.