Thirty-seven preterm infants with idiopathic respiratory distress syndrome were prospectively studied for the effect of the early closure of patent ductus arteriosus with indomethacin on the course of idiopathic respiratory distress syndrome. Serial retrograde aortograms were performed in all infants in order to visualize the ductus arteriosus, apart from three patients, who died early and were evaluated aortographically only once. The ductus was initially open in 27 infants and closed in 10 infants. The infants with open ductus arteriosus were randomly divided into two groups. The first group consisted of 13 infants, in whom the ductus was closed with indomethacin at a median age of 18 hours. The other 14 infants served as controls. Total time on assisted ventilation and duration of exposure to additional oxygen were significantly shorter in medicated infants than in controls. Oxygenation of infants with an initially closed ductus was better from birth and duration of their ventilatory assistance and oxygen exposure were shorter than in infants with initial ductal shunting. The data suggest that the early closure of the patent ductus arteriosus with indomethacin in distressed preterm infants has a favourable effect on the course of idiopathic respiratory distress syndrome.