We used pulsed Doppler ultrasound to determine the flow dynamics of the aorta and patent ductus arteriosus (PDA) in 21 infants (ages 1 to 16 days) with PDA and aortic atresia (n = 15) or aortic stenosis (n = 6). The flow within the PDA was recorded in 19, and was right-to-left during systole in all. There was a diastolic left-to-right PDA shunt in 11 patients with aortic atresia and in three with aortic stenosis, and the shunt was associated with large (3 to 11 mm in diameter) interatrial communications. In two patients with aortic atresia and three with aortic stenosis, however, the diastolic PDA shunt was from right to left, and the interatrial communications were small (0 to 2 mm in diameter). The right-to-left diastolic PDA shunting may be best explained by the relative pulmonary (high with left-sided inflow obstruction and a small interatrial communication) and systemic resistances. All patients with aortic atresia and three with aortic stenosis had retrograde systolic flow in the transverse aortic arch, probably resulting from inadequate left ventricular output. Antegrade diastolic flow in the transverse aortic arch toward the PDA was observed in all infants with aortic atresia and a left-to-right PDA shunt. Ascending aortic flow was recorded in 11 patients with aortic atresia, and was retrograde during diastole in each, the result of coronary perfusion. Application of pulsed Doppler ultrasound can lead to a better understanding of the hemodynamics and physiology of patients with congenital cardiovascular disease.