Indomethacin has long been used to treat patent ductus arteriosus but it is associated with a relatively high risk of adverse effects; recent evidence suggests that ibuprofen is effective and may be safer. In a randomised trial to compare the efficacy and safety of ibuprofen and indomethacin in the treatment of patent ductus arteriosus, 144 infants received three doses of ibuprofen lysine (10, 5 and 5 mg/kg) at 24-hour intervals or indomethacin 0.2 mg/kg at 12-hour intervals. Ductal closure occurred in 70% of children treated with ibuprofen and 66% of those given indomethacin on the first treatment (P = 0.41). Nineteen children underwent surgical ligation, equally distributed between the treatment groups (P = 0.81). Urine production was significantly greater than in children given indomethacin from day 3 to day 7 and the serum creatinine concentration was significantly lower from day 7. Ductal closure was associated with higher serum concentrations of ibuprofen and a concentration of 10-12 mg/l appears to be the minimum level for efficacy. In a randomised, placebo-controlled, double-blind trial of prophylaxis with ibuprofen, the rate of patent ductus arteriosus associated with ibuprofen was 19% compared with 42% with placebo. Urine output was comparable in the two groups except on day 1, when it was significantly lower among infants given ibuprofen. Ibuprofen is therefore as effective as indomethacin in the treatment of patent ductus arteriosus, and effective as prophylaxis, in premature infants.