Certain morphologic aspects of atrioventricular septal defects ('endocardial cushion defects,' 'atrioventricular canal malformations') remain controversial. It is still not clear which precise lesions should be placed in this category. For example, is an 'isolated' cell of the mitral valve or a perimembranous inlet ventricular septal defect to be so described? It is also not fully accepted that the left atrioventricular valve in these lesions bears little resemblance to a morphologically mitral valve. We have investigated these problems by both observation and mensuration. We determined the junctional circumference of the left atrioventricular valve leaflets and the ventricular dimensions in 130 atrioventricular septal defects (95 with common valve orifice and 35 with separate right and left atroventricular orifices); in 50 hearts with perimembranous ventricular septal defects (20 extending into the inlet septum and 30 with outlet or trabecular extensions); in seven hearts with isolated cleft of the mitral valve, and in 10 normal hearts. All specimens came from the cardiopathological collection of Children's Hospital of Pittsburgh. The measurements showed conclusively that the atrioventricular septal defects were all directly comparable irrespective of the detailed morphology of the atrioventricular valve or valves. The group of atrioventricular septal defects was totally discrete as compared with all the other specimens that had normal atrioventricular septation. The left atrioventricular valve in atrioventricular septal defects is basically a three-leaflet valve which differs from the normal mitral valve in terms of its leaflet, its chordal support, and the arrangement of its papillary muscle. Its only similarity with the normal mitral valve is that it resides in the morphologically left ventricle and guards the left atrioventricular junction.