A Prospective study was done about incidence and treatment of hemodynamically significant' patent ductus arteriosus in premature infants admitted to neonatal intensive care unit of Ewha University Hospital from Feb. 1984 for one year. The results were as follows; 1) Among 46 infants with birth weight less than 2,000gm and gestational period 36 wks, 15(32.6%) developed a hemodynamically significant PDA. 2) The incidence of respiratory distress syndrome was six times higher in PDA group than that of non-PDA group(73.3% vs 12.9%). 3) The need for ventilatory assistance was five times higher in PDA group than that of non-PDA group(80% vs 16.1%). 4) The mortality rate was 40% in PDA group, 15% in non-PDA group. 5) The ratio of left atrium to aorta(LA/AO) in M-mode echocardiography was significantly higher in PDA group than that of non-PDA group(1.21+/-0.18 vs 0.93+/-0.23). 6) Indomethacin treated with usual medical therapy resulted in ductal closure in 80%, 20% with usual medical therapy only. 7) The mean LA/AO ratio was significantly decreased after indomethacin treatment(1.15+/-0.13 vs 1.21+/-0.18). 8) The change of the value of plasma sodium and potassium was not significant during indomethacin therapy, whereas BUN, creatinine value was increased during treatment, but that was reversible.