Literature reference of the time of spontaneous closure of patent ductus arteriosus is based on invasive or autopsy studies of selected patients; such patients may not have been normal. Since pulsed Doppler echocardiography permits sensitive and specific diagnosis of PDA, 50 normal newborn infants, appropriate in size for gestational age, were examined by PDE for evidence of PDA. A PDA was diagnosed when diastolic ductal flow into the main pulmonary artery could be recorded, using standard precordial transducer placement, or when continuous turbulent right pulmonary artery flow could be recorded using the suprasternal approach. Infants were examined as soon as possible after birth, and twice daily thereafter until PDA could no longer be detected. Standard M-mode echocardiographic measurements were made at the initial and final examinations. In 92% of infants, PDA was detected at the time of initial examination. Of these, PDA could no longer be detected in 42% by 24 hours of age, in 78% by 40 hours, in 90% by 48 hours, and in all by 96 hours of age. M-mode echocardiographic evidence for a significant left-to-right PDA shunt was not present in any patient. In 28% of patients with PDA, a murmur was heard at some time prior to PDA closure. Of infants 36 to 38 weeks' gestation, five of six had PDA closure later than 32 hours of age; six of seven infants of 40 weeks' gestation had closure of PDA prior to 32 hours of age. There was no clear association between time of PDA closure and birth weight, maternal medication, or duration of labor. We conclude that ductal patency is common in the normal newborn infant, and that spontaneous closure may not occur until considerably later than previously believed.