We described previously that the degree of deficiency of the ventricular septum (scooping) affects electrocardiographic findings in atrioventricular septal defect with separate right and left atrioventricular orifices. The aim of this study was to clarify the clinicomorphological relationships of the degree of scooping, the precise morphology of the atrioventricular valve, and electrocardiographic findings in hearts having a common atrioventricular orifice. In 63 patients with usual atrial arrangement, the outlet, scoop and inlet dimensions of the ventricular septum were measured by biplanar cineangiography and were compared with the body surface area. The scoop/outlet ratio and inlet/outlet ratio were also compared with the body surface area, atrioventricular valvar morphology, electrocardiographic findings, and severity of regurgitation of the atrioventricular valve. The results showed that the three individual dimensions of the septum correlated significantly with the body surface area, even though the scoop/outlet ratio and inlet/outlet ratio did not. A correlation was found between the scoop/outlet ratio and the degree of left axis deviation on the electrocardiogram (r=0.74, p<0.001). In 16 patients with division of the inferior bridging leaflet, there was a lower scoop/outlet ratio than in the remaining 37 with an undivided bridging leaflet (p<0.01). Of 24 patients with counterclockwise rotation in the horizontal loop of the vectorcardiogram, 22 had an undivided inferior bridging leaflet (p<0.01). The morphology of the superior bridging leaflet, in contrast, was not associated with the scoop/outlet ratio, and neither was the severity of valvar regurgitation. We conclude that the degree of scooping of the ventricular septum is related not only to electrocardiographic findings, but also to the morphology of the inferior bridging leaflet in atrioventricular septal defect with common atrioventricular orifice.