OBJECTIVE: To determine the prognosis of supravalve aortic stenosis into early adult life and the factors affecting this prognosis. DESIGN: 81 patients with supravalve aortic stenosis were followed for a median duration of 8.3 (range 1 to 29) years. PATIENTS: 40 patients (49.4%) had Williams' syndrome, 18 (22.2%) familial supravalve aortic stenosis, 18 (22.2%) sporadic supravalve aortic stenosis, and five (6.2%) other syndromes. Nineteen patients had additional levels of left ventricular outflow tract obstruction. RESULTS: 47 patients (58%) underwent operation; 20% within a year of presentation. Multivariable analysis predicted that 88% of patients would undergo intervention within 30 years of follow up. The chance of intervention was increased by more severe aortic stenosis at presentation and the presence of multilevel obstruction in patients with sporadic supravalve aortic stenosis. Three deaths occurred before operation and 13 within a month of operation. Ten (62.5%) of the postoperative deaths were in patients with multilevel obstruction. Predicted survival 30 years after presentation was 66%. Risk factors for survival were age and severity of aortic stenosis at presentation. Multilevel obstruction did not emerge as a significant risk factor for death because of the high association with the severity of stenosis at presentation. 74% of survivors had mild or insignificant stenosis at follow up. CONCLUSIONS: Long-term survival is related to age and the severity of aortic stenosis at presentation. Most patients will require intervention, and most survivors will have mild stenosis.