A double-blind controlled trial of intravenous indomethacin therapy was performed using a group of 55 premature infants (27 placebo, 28 indomethacin) with a significant persistent ductus arteriosus. Indomethacin administration at a mean postnatal age of 8.9 days was followed by a significant effect on PDA in 89%; 75% of successes were attributable to indomethacin and 25% to spontaneous effects, an improvement by indomethacin of 86% in infants not undergoing spontaneous improvement. The short-term side effects of indomethacin were transient; urinary output and serum sodium concentration decreased and serum potassium concentration increased. Indomethacin administration was associated with a decreased need for assisted ventilation and a decreased need for surgical closure of PDA. There was no significant difference between the placebo and indomethacin groups in mortality and bronchopulmonary dysplasia morbidity. The infants who developed BPD had higher RDS scores and lower PO2 values, requiring higher FIO2s within four hours of birth than those who did not develop BPD, indicating a more severe underlying pulmonary disability present birth.