Cross-sectional echocardiograms of 18 neonates and infants with coarctation of the aorta and ventricular septal defect were retrospectively assessed. With a combination of subcostal and precordial cuts the site and anatomical relations of the defects were determined. In one case there was a muscular trabecular ventricular septal defect. Three had a malalignment defect with associated left ventricular outflow tract obstruction. In two there was a doubly committed subarterial defect with associated malalignment of the point of continuity between the aortic and pulmonary valves and the crest of the trabecular septum. Twelve cases had a perimembranous defect, with varying degrees of extension into the inlet, trabecular, or outlet septum. In this group 10 had associated aortic override, with varying degrees of left ventricular outflow tract narrowing. Abnormal insertion of the tricuspid valve was observed in 10 cases, such that it partly obscured the ventricular septal defect. Thus in most cases of coarctation with ventricular septal defect, the morphology of the ventricular septal defect and ventricular outflow tracts is such that left ventricular ejection is directed towards the pulmonary artery rather than the aorta.