This study evaluates the course of supravalvular aortic stenosis (SVAS)-associated right ventricular outflow tract (RVOT) obstruction and the results of surgery in children. We reviewed the medical records of 24 patients diagnosed with SVAS at initial echocardiographic examination or during the following period of RVOT obstruction. Very mild SVAS was defined as a transvalvular Doppler peak systolic instantanous gradient (PSIG) less than 25 mmHg, mild stenosis as 25-49 mmHg, moderate stenosis as 50-75 mmHg, and severe stenosis as more than 75 mmHg. The mean age of the patients was 3.1 +/- 2.9 years (range, 7 days to 12.7 years), and 18 of the patients (72%) were male. Fifteen patients had Williams' syndrome. Seventeen patients (71%) were followed for a mean of 5.2 +/- 3.8 years (range, 7 months to 13.5 years). Among 17 patients with complete follow-up records, 1 (6%) had very mild, 5 (29%) mild, 3 (18%) moderate, and 3 (18%) severe aortic stenosis at initial echocardiographic examination. In a newborn patient with mild pulmonary valvular stenosis. SVAS became evident after 2 months and progressed rapidly. Supravalvular aortic stenosis was very mild in 4 patients (23%), mild in 3 (18%), moderate in 3 (18%), and severe in 7 (41%) at last echocardiographic examination. Of 17 patients who were followed, 11 (65%) had RVOT obstruction at initial echocardiographic examination. RVOT obstruction disappeared in 5 patients, regressed in 1 patient, and appeared in 1 patient over the follow-up period. Four patients underwent operation. It appears reasonable that patients with very mild and mild stenosis should be followed medically every 1 or 2 years and patients with moderate stenosis once a year. Newborns with SVAS should be followed for rapid progression of SVAS. In some patients, RVOT obstruction may disappear, and SVAS may develop in others with RVOT obstruction. Patients with RVOT obstruction (at the valvular, supravalvular, or peripheral pulmonary arterial level) should be evaluated carefully for development of SVAS at follow-up.