Over a 4-year period, 63 premature babies presenting with patent ductus arteriosus were treated with indomethacin at two different dosage levels (0.1 and 0.2 mg/kg). For all infants the permanent closure rate was 68% (56% with 0.1 mg/kg and 75% with 0.2 mg/kg - n.s.) and the positive response rate (i.e. permanent and transient closure) 84%. There was no difference between the two dosages in the incidence of side-effects, particularly on diuresis, and in mortality and morbidity rates; the overall mortality rate was 31%. Pharmacokinetic studies performed in 51 cases did not show any difference related to indomethacin dosage. The changes observed after 2 or 3 doses resulted from improvement in haemodynamics. This study confirms the effectiveness of indomethacin in the treatment of patent ductus arteriosus and suggests an advantage for the 0.2 mg/kg dose.