In the treatment of cyanotic heart diseases in neonates who are ductus dependent, the small size of the pulmonary artery and/or subclavian artery often precludes construction of a sufficient Blalock-Taussig shunt. To avoid an emergency operation and to obtain pulmonary and subclavian arteries of adequate size (3 mm), a low dose of prostaglandin E1 was given to 16 neonates for a mean of 47 days (range 26 to 65 days), and a modified Blalock-Taussig shunt operation was performed in neonates whose mean age was 50 days. The pulmonary artery grew significantly (mean 4.5 mm) and the mean diameter of the subclavian artery was 3 mm, which was large enough for a good shunt at the time of operation. There were two late deaths and one operative death. All surviving patients shunts at the mean follow-up period of 22 months. The shunt was constructed on the same side as the ductus arteriosus so that the ductus could be ligated at the same operation in selected cases; the ductus may not close after discontinuation of prostaglandin E1 infusion and may cause uncontrollable congestive heart failure because of excessive pulmonary blood flow, particularly in patients with a univentricular heart. The ductus, dissected in nine patients, was enlarged and elongated in all and was ligated in four patients without complication.