The purpose of this study is to determine the incidence of reverse aortic arch flow in newborn infants with pulmonary hypertension, and no other identifiable cause for flow reversal, being considered for extracorporeal membrane oxygenation (ECMO) 2) compare the left ventricular output, and 3) outcomes, of these infants, with those in the group of patients with normal arch flow. We reviewed the medical records and echocardiograms of all infants referred to our institution for extracorporeal membrane oxygenation support for pulmonary hypertension, between August 1994 and April 1996. Neonates with pulmonary hypertension and reverse aortic arch flow had significantly lower left ventricular output (p = 0.005), and had significantly higher mortality (p = 0.04), than those with normal aortic arch flow. They required significantly higher ventilatory support (p = 0.01) and tended to need more inotropic support. Reverse aortic arch flow in newborn infants with pulmonary hypertension, and no other cause for the reverse flow, indicates significantly decreased left ventricular output, and may be a marker for a more difficult clinical course and poor prognosis. In this group of patients with pulmonary hypertension, the survival was significantly higher in infants with no PDA or with pure left to right shunt across a PDA.