In 1983, a US National Collaborative Study (NCS) proposed criteria for the diagnosis of hemodynamically significant patent ductus arteriosus (PDA) in premature infants with respiratory distress syndrome (RDS), but the widespread use of pulsed Doppler cross-sectional echocardiography (PD-CSE) in neonatal intensive care units has made direct assessment of the ductus possible thus providing more timely therapy. We have compared the results in 30 premature infants with severe RDS, assessed according to the guidelines of the US NCS, with those in 51 infants whose PDA was diagnosed by PD-CSE. Together with a significant reduction in the age at treatment (7.8 +/- 3.9 vs 2.4 +/- 1.1 days), there was a reduced dependence on artificial ventilation (14.8 +/- 11.0 vs 7.8 +/- 2.7 days), a reduction in the number requiring surgical ligation of PDA (9 vs 2), a decreased incidence of bronchopulmonary-dysplasia (BPD) (40% vs 16%), and a reduction of unfavorable outcome of treatment (death or BPD) (76% vs 49%).