Abstract. Sixteen premature infants were treated with indomethacin, either orally or intravenously, in order to close a symptomatic patent ductus arteriosus requiring mechanical ventilation. Positive effect was obtained in 4 of the 10 orally treated patients and in 1 of the 6 with intravenous administration. Immediate side effects were impairment of renal function, abdominal distension, and restlessness.Although the oxygen administration was guided using a transcutaneous oxymonitor, three severe cases of retrolental fibroplasia occurred. As indomethacin is a nonselective inhibitor of all prostaglandins, it might also cause a constriction of the retinal arteries, and as such be a factor in the development of retrolental fibroplasia in small premature infants.