After Norwood's initial report of successful first-stage palliation of hypoplastic left heart syndrome in neonates, an improved understanding of both the pathologic anatomy and physiology of this common condition has led to improving surgical results. Twenty-five neonates have been treated by modifications of the Norwood procedure, and the early mortality between January, 1984, and July, 1985, has been 24% (six deaths). Two premature neonates weighing less than 2 kg died, as did two neonates who could not be resuscitated with prostaglandin E1 and had cardiac arrests immediately preoperatively. There have been four late deaths. The frequent occurrence (80%) of a juxtaductal coarctation or coarctation shelf necessitates alteration of surgical technique. Failure to do so will be reflected in an early persistent postoperative metabolic acidosis with an arterial oxygen tension greater than 35 to 40 mm Hg in the operating room or greater than 40 to 45 mm Hg in the intensive care unit. A similar acid-base derangement will be seen if the systemic-to-pulmonary shunt is excessively large. Careful ventilatory and pharmacologic management of the ratio of pulmonary to systemic vascular resistance is an essential part of the perioperative management of these neonates with two parallel competing circulations. Late postoperative catheterization findings have emphasized the importance of a wide atrial septectomy, because restrictive atrial septal defects have developed in a number of children, despite partial surgical atrial septectomy. Central pulmonary artery reconstruction must be carefully performed to avoid distorted hypoplastic pulmonary arteries that would be unsuitable for a second-stage modified Fontan procedure.