Background It is uncertain whether proximal pulmonary artery (PA) obstruction exists soon after birth and whether its progress relates directly to postnatal ductal constriction in congenital heart disease and obstructed pulmonary flow. Methods Serial morphometric analyses of the PA branches by echocardiogram were performed in 28 patients (mean age at initial study 2.5 days) until severe constriction of the ductus occurred (mean age 47 days). These patients were divided into 2 groups by subsequent angiographic or postmortem confirmation; 10 with proximal PA obstruction (group 1) and 18 without obstruction (group 2). Results At the time of initial examination, the mean indexed diameter of the proximal PA on the side of the ductus arteriosus in group 1 was significantly smaller than that on the contralateral side (5.2 ± 0.7 versus 9.0 ± 0.7 mm/BSA^0.5 , P < .001) or that in group 2 (8.0 ± 0.4 mm/BSA^0.5 , P < .001). In group 1, 8 patients had a proximal PA index on the ductal side ≤ 5.5 mm/BSA^0-5 , which was less than those of any group 2 patients. After severe constriction of the ductus, the proximal PA index on the ductal side further decreased only in group 1 (P < .01). Conclusions These data indicate that unilateral obstructive lesion of branch PA is present shortly after birth and its progression relates directly to ductal constriction. Neonates with branch PA obstruction can be identified on their initial echocardiogram as having a proximal PA index on the ductal side ≤ 5.5 mm/BSA^0.5 . (Am Heart J 1998;136:837-43.)