The morphologic characteristics of ventricular septal defect (VSD) and left ventricular outflow tract were studied in 45 hearts with VSD and coarctation of the aorta (C of A). Forty-one VSDs were classified into 1 of 4 categories. The first category includes 19 central muscular VSDs (43%), among which 3 hearts had the architecture of a spontaneously closed defect. The second category includes 10 perimembranous inlet defects (23%) with overlying tricuspid valve. The third category is formed by 8 perimembranous VSDs with leftward outlet septal malalignment (18%) and the fourth category includes 4 subarterial VSDs (8%) in which leftward malalignment of a deficient outlet septum results in subpulmonary localization of the defect. In addition to the main categories, 1 membranous, 1 perimembranous trabecular, 1 apical muscular and 1 doubly committed subarterial defects were identified. The left ventricular outflow tract was assessed as normal in only 2 cases. In 43 cases the left ventricular outflow tract was compromised by 1 or more of the following anomalies: anterolateral muscle bundle, anteroseptal twist, bicuspid aortic valve or a leftward malaligned outlet septum. These findings support the hypothesis of C of A being hemodynamically induced by anomalies of the left ventricular outflow tract. Previous studies have established that muscular, membranous and perimembranous inlet defects are prone to close spontaneously, in contrast to malalignment and subarterial VSDs. The present study shows that 70% of the VSDs belong to types characterized by a high incidence of spontaneous closure.