The hemodynamics and patency of ductus arteriosus (PDA) in 51 infants were examined clinically and by echocardiography during the first or second day of life. Gestational ages ranged from 27 to 41 wk (34.0 +/- 3.6); birth weights ranged from 1070 to 4320 g (2256 +/- 923). Ductal flow was apparent by Doppler echocardiography in 24 of 51 infants but clinical examination disclosed only nine of these. Ultrasound is superior to clinical examination in diagnosing both symptomatic and asymptomatic PDA. Doppler ultrasound has the additional advantage of evaluating the size and relative importance of the ductal flow.