The morphologic characteristics of ventricular septal defect (VSD) was studied in 50 hearts with complete transposition of the great arteries. Except for 1 membranous and 1 subarterial defect, all VSDs could be classified into 1 of 4 categories. Group A included 14 specimens with a perimembranous defect extending into inlet and trabecular septum with rightward malalignment of the outlet septum; in this group aortic arch anomalies (6 hearts) and tricuspid valve abnormalities (7 hearts) were frequently present. Group B comprised 13 hearts with perimembranous defects extending into the inlet septum; 11 hearts in this group also had an anomalous mitral valve. Group C included 13 hearts with muscular inlet defects, with signs of spontaneous closure by marginal fibrosis in 10 cases. Group D included 8 hearts with muscular infundibular defects; in 6 of these hearts the outlet septum was malaligned to the left, causing different degrees of subpulmonary stenosis. A small second VSD was present in 5 hearts. Thus, the spectrum of VSD in transposition of the great arteries is different from that of isolated VSD.