We studied the change in pulmonary arterial diameter under prostaglandin E1 (PGE1) therapy in infants with ductus-dependent cyanotic heart disease (pulmonary atresia). Ten infants undergoing administration of PGE1 for more than 2 weeks were selected for this study. A classic Blalock-Taussig (BT) shunt was performed in seven patients and a modified BT shunt in three. The shunt was successful in all patients. The internal diameters of the right and left pulmonary arteries were measured before and after the start of PGE1 therapy using two-dimensional echocardiography. The pulmonary arterial index (PA index) was calculated to correct the diameters for body surface area. Both pulmonary arteries enlarged during the first week of PGE1 therapy in all 10 patients. Further increases in the two diameters were observed even after the first week of treatment in nine patients. Both the mean diameters at 2 weeks after the start of PGE1 were about 50% larger than the initial diameters (right; increased from 3.1 to 4.7 mm; left; increased from 3.0 to 4. 4 mm). Of the eight patients given PGE1 for more than 3 weeks, four showed no changes in pulmonary arterial diameters after the first 2 weeks and the remaining four showed a slight increase. PA indexes also showed a rapid increase during the first 2 weeks and no significant change thereafter. We suggest that, in infants with pulmonary atresia and small pulmonary arteries, the optimal pulmonary artery size for BT shunt insertion is achieved after 2 weeks of PGE1 infusion, with no further significant increase in size being observed after this time.