During a 19-month period, 23% of preterm infants had symptomatic patent ductus arteriosus (PDA). Intravenous indomethacin was administered to 67 infants with successful closure in 91% of the patients and in 83% of those with birth weights less than 1,000 g. No differences were found between the number of doses required and birth weight, gestational age, or age at initial therapy. Transient alterations in renal function were common after therapy, but mild renal failure occurred in only four infants. The incidence of intraventricular hemorrhage and necrotizing enterocolitis in treated infants was similar to that in the total preterm population. Cicatricial retrolental fibroplasia developed in one patient. Bronchopulmonary dysplasia occurred more frequently in infants with PDA, but a decreased incidence was found after early ductal closure. Intravenous indomethacin administration successfully effects ductal closure without major complications in preterm infants.