We have identified 12 patients with endocardial cushion defect and marked under-development of the left ventricle. Most of these patients had significant obstructive anomalies of the aortic arch. Pathologically, the left ventricle was very small, but without endocardial fibroelastosis, significant subaortic narrowing was evident, resulting from maladherent atrioventricular valve tissue to the left ventricular outflow tract, and the left ventricular posterior leaflet component of the atrioventricular valve was both grossly deficient and dysplastic. In all 12 a large ostium primum atrial communication was present, but the ventricular contribution to the defect was small. The clinical presentation of severe, intractable congestive heart failure in these neonatal patients would suggest the diagnosis of typical hypoplastic left heart syndrome. Important departures in some of these patients included a leftward, superior and counterclockwise frontal vector on the electrocardiogram and a widely split second sound. Selective biplane angiocardiography will reveal the underdeveloped left ventricle, with an elongated, fixed, 'gooseneck' deformity of the left ventricular outflow tract, severe 'mitral' regurgitation, and a small ascending aorta. Selective right ventriculography will opacify the very much larger right ventricle and pulmonary artery. Finally, these cases broaden the concept of double inlet right ventricle and exaggerated displacement of the atrioventricular canal towards the bulbus cordis.