We examined the cross-sectional echocardiographic findings of 171 patients with atrioventricular septal defects. The echocardiographic findings were confirmed by angiography, surgery and/or autopsy. The echocardiographic findings determined whether the common atrioventricular junction was guarded by a common valve or separate right and left valves. In addition, we were able to judge whether the bridging leaflets were related to the septal structures so as to permit both interatrial and interventricular communications [127 cases] or whether the interatrial communication ("ostium primum atrial septal defect") [43 cases] or an interventricular communication [1 case] existed in isolation. Defects existing with a common atrioventricular valve could be further classified as having minimal bridging of the antero-superior leaflet (Rastelli Type A [113 cases]); intermediate bridging (Rastelli Type B [3 cases]); or extreme bridging (Rastelli Type C [11 cases]). Of the patients with Down's syndrome, 9 had separate right and left valves while 66 had a common valve, all the latter existing in the setting of minimal bridging of the antero-superior leaflet. In the overall group, there were 9 cases having an unbalanced ventricular mass, 5 with right ventricular dominance and 4 with left dominance. Other associated defects were common. The echocardiographic findings were supplemented by pulsed Doppler examination. Atrioventricular valve insufficiency, when mild, was frequently demonstrated only in the right atrium just above the leaflets of the atrioventricular valve. When there was more severe valve insufficiency the regurgitant jet could be detected for greater distances behind the atrioventricular valve and in either or both atria, but more frequently in the left atrium. As might be anticipated in view of the complexity of the lesion, a combined imaging approach yielded the most accurate results.