To find out whether there is an underestimated severity of narrowing in obstructive lesions of the pulmonary outflow tract in the first day of life, Doppler measurement of the pulmonary outflow tract gradient and estimation of the pulmonary artery systolic pressure in 15 neonates with pulmonary outflow tract obstruction with a variety of associated lesions were studied in the first twenty-four hours of life (mean thirteen hours, range six to twenty-four) and repeated at the age of twenty-six to seventy-two hours (mean forty-nine). The maximal instantaneous gradient across the pulmonary outflow tract obstruction (22.4 +/- 9.0 mmHg) in the first day of life was significantly lower than at the repeated study (61.0 +/- 19.4 mmHg, (P < 0.001). In 12 patients with patent ductus arteriosus, transductal gradient increased significantly at the repeated examination (19.6 +/- 9.3 vs 48.2 +/- 6.1 mmHg, P < 0.001). The pulmonary outflow tract gradient in these 12 patients was 22.3 +/- 8.4 vs 62.9 +/- 21.1 mmHg, (P < 0.001). The severity of the pulmonary outflow tract obstruction was underestimated in the first twenty-four hours of life. Low gradient across pulmonary outflow tract and low transductal gradient in the first day of life mirrored high pulmonary arterial systolic pressure. At the repeated study the increase in transductal gradient and the increase in the pulmonary outflow tract gradient more accurately represented the severity of pulmonary outflow tract obstruction owing to the decline in the systolic pulmonary arterial pressure.