This study examined the response of the patent ductus arteriosus (PDA) to intravenous Indomethacin using serial two dimensional and Doppler echocardiography and documented the complications associated with therapy. Thirty-six preterm neonates who were oxygen and ventilator dependent were studied when they were aged 3-7 days. The PDA initially closed in 22 (61%) and constricted in seven (19%) of the infants. It was non-responsive in five (14%) and the treatment was stopped because of complications in two (6%). Only three (43%) of seven neonates given a second course had PDA closure. In the 25 instances where there was PDA closure following Indomethacin, re-opening was documented echocardiographically on three (12%) occasions. Overall, Indomethacin therapy was successful in 29 (81%) neonates, PDA ligation was required in four (11%) and three died from unrelated causes. Three (8%) neonates developed major complications: multiple gastric perforations in the first, focal ileal perforation in the second, and necrotizing enterocolitis in the third. Treatment failure, PDA ligation and major complications occurred exclusively in neonates less than 28 weeks gestation. In view of the relatively low efficacy and high major complication rate in these extremely preterm infants, a randomized clinical trial needs to be conducted using two dimensional and Doppler echocardiography to allow accurate assessment of the PDA response to intravenous Indomethacin.