Among 111 hearts with so-called "isolated" ventricular septal defect, 18 specimens were found to have a subaortic perimembranous defect with an overriding aortic valve but without pulmonary stenosis. The ventricular architecture was characterized by several abnormalities. A constant finding was the wide right ventricular outflow tract. The outlet septum had its normal continuity with the septomarginal trabecula, but its parietal extension was located relatively far anteriorly. Part of the aortic valve thus inserted to the right ventricular component of the ventriculo-infundibular fold in the gap between the outlet septum and the tricuspid valve. Left ventricular abnormalities comprised mitral valve anomalies in all cases. There was an anteroseptal twist (leftward thickening of the anterior part of the ventricular septum) in 16 cases. A bicuspid aortic valve and/or malformed cusps were observed in 4 cases. Because of the linking phenomenon of aortic override, we also examined 10 hearts with tetralogy of Fallot and, in the latter, such abnormalities were not found. Our observations indicate that this seemingly simple type of defect is part of a complex malformation involving both septation and valve formation. Awareness of the existence of the architectural abnormalities might make them accessible for echocardiographic diagnosis. It was noteworthy that 11 of the 18 patients had chromosomal abnormalities, 9 of them presenting with trisomy-18.