Sixteen patients with endocardial cushion defects were studied by echocardiography and ultrasonocardiotomography. Three characteristic findings, common to both complete and partial forms, were observed. They were the ostium primum defect of the interatrial septum, the left ventricular outflow narrowing and the atrioventricular valve abnormalities which are the anatomic bases of endocardial cushion defects. Several additional findings were observed in patients with complete form. Most strikingly, the anterior leaflet echo locating in the left ventricle had the direct connection to that in the right ventricle without the interposition of the interventricular septum by echocardiogram. Ultrasonocardiotomogram, on the other hand, demonstrated beautifully the common anterior leaflet and the high inteventricular septal defect. There were, in addition, the absence of the paradoxical motion of the interventricular septum and the diffuculty in detecting the interatrial septum. The intact interatrial septum continuous with the anterior mitral leaflet was demonstrated in patients with the interventricularseptal defect of atrioventricular canal type, in which there were the typical abnormalities in the atrioventricular valves and the left ventricular outflow narrowing as well. This study indicates that ultrasonic examination may provide a direct, yet noninvasive method in the diagnosis of various types of endocardial cushion defects.