Most natural history data regarding pulmonary stenosis (PS) were obtained from cardiac catheterization studies over 15 to 20 years ago. Selection bias in these studies often excluded patients with mild disease and infants. Today, Doppler echocardiography allows accurate serial assessments of stenotic lesions in patients of all ages. This study evaluates the natural history of PS utilizing serial Doppler examinations in the pediatric population. A total of 147 patients with PS and serial echocardiographic data were identified. Age at initial echocardiogram ranged from 2 days to 15 years, with a mean follow-up of 2.4 years. Sixteen of 56 patients (29%) initially evaluated within 1 month had a > or = 20 mm Hg increase in their peak systolic pressure gradient. Only 7 of 89 patients (8%) initially evaluated over 1 month had a > or = 20 mm Hg increase. Eleven of 40 newborn infants (28%) with mild obstruction had progression to moderate or severe PS compared with 10 of 68 patients (15%) initially evaluated over 1 month. Moderate PS in the newborn was also more likely to progress compared with older children. Of the 16 newborns with > or = 20 mm Hg increases, 8 developed the increase in < or = 6 months. In contrast, no patient aged >2 years whose initial gradient was <50 mm Hg developed severe PS. Mild PS may not be static, particularly in young infants. Progression in this age group occurs more often and more rapidly than in older infants and children.