A characteristic feature of atrioventricular septal defects is a deficiency of the inlet part of the ventricular septum that results in a 'scooped out' appearance. The depth of the scoop in relation to the disposition of the atrioventricular valves has been debated. To clarify the relation between the morphology of the ventricular septum and the disposition of the atrioventricular valves, we quantified these anatomic features in 151 hearts at autopsy to determine whether those features identified particular groups within the overall lesion. We found that 137 hearts had left atrioventricular valves with three leaflets. The left valve in the other 14 hearts exhibited a dual orifice, a two-leaflet or one-leaflet arrangement, or was imperforate. These anomalies could be analyzed in terms of a sequence of diminishing formation of the commissures. Also, three-leaflet valves displayed a variability in which the angular size of the mural leaflet correlated negatively with that of the inferior leaflet. In some of the hearts with a common atrioventricular orifice, the bridging leaflets did not meet over the ventricular septum, thus creating a 'gap'. The mural leaflet's angular size corresponded to a deficiency of the combined inferior-mural leaflet complex. Hearts with an abnormal disposition of the left atrioventricular valve had the ventricular septum 'scooped' to a greater extent than those with a common orifice, although most had separate right and left atrioventricular orifices.