Delivery of right ventricular blood into localized segments of the systemic arterial system depends upon "reversal of flow" through a patent ductus arteriosus. Obstruction of the aorta, either in the form of coarctation or interruption of the arch, is frequently an associated lesion. Variations in the site of origin of the subclavian arteries underlie variations in perfusion to the arms. Delivery of right ventricular blood to localized systemic arterial segments is readily evident in angiocardiograms and may be associated with differential cyanosis. The latter feature is not always recognized clinically, however. Lack of identification of differential cyanosis is explained by the fact that a ventricular septal defect and associated left to right shunt are commonly present when there is an anatomic basis for differential cyanosis.