Sixty-four preterm infants (birth weight 1,393 +/- 483 g g and gestational age 30.1 +/- 2.7 weeks) developed a symptomatic ductus arteriosus, and were treated with intravenous indomethacin. Mechanical ventilation was needed by all of them, and 98% had IRDS. Pharmacologic ductus closure rate was 87%. Sixty-four per cent of patients had a permanent closure after first trial, and 74% after a second course of the drug. Ductus closure rate was found to be greater in patients with higher birth weight and gestational age. It was also higher in patients treated in the first week of life. Closure rate was higher in this group of patients treated with intravenous indomethacin, than in a previous group of a similar birth weight and gestational age, who received the drug by oral route (87% versus 64%). Incidence of necrotizing enterocolitis was found to be similar in both groups. Follow-up data showed that four infants (8.5%) had severe neurological sequelae, and another three cases (6.4%) developed severe respiratory problems.