The theory is advanced that increased pulmonary vascular resistance, resulting in a state of fetal circulation, with right-to-left shunting through the ductus arteriosus, is the main reason that many patients do not survive after repair of a diaphragmatic hernia. Three patients are presented (who, by Raphaely's criteria, were destined for a fatal outcome) in whom the ductus was ligated, and vasodilator drugs were infused into the pulmonary artery. All three demonstrated definite improvement in oxygenation. Two expired after 6 days, one of whom was found at autopsy to have intestinal volvulus and gangrene, and the other multiple plumonary emboli. One case, so managed, survived. The suggestion is made that pulmonary hypoplasia is not the main reason for the high mortality rate after diaphragmatic hernia repair, and that additional laboratory and clinical investigation of the pulmonary circulation may lead to significant improvement in results.