Six preterm infants with a persistent ductus arteriosus, who failed to improve with conventional medical management, received indomethacin in an attempt to close the duct pharmacologically. All infants weighed less than 1000 g when the drug was administered. All showed a transient response to indomethacin; however, no infant demonstrated a permanent response, even though 5 of the 6 received multiple doses. One of the patients developed a severe episode of anuria, uraemia, and gastrointestinal bleeding. Of the 6 infants, 3 underwent subsequent successful surgical ligation of the ductus. It is suggested that the extremely preterm infant may be a 'poor responder' to indomethacin. Since the side effects of indomethacin may be life-threatening, it may be wise to consider surgical ligation in lieu of indomethacin administration in these infants.