OBJECTIVE--To determine the incidence and prognosis of congenital aortic valve stenosis in the five Health Districts of Liverpool that make up the Merseyside area. DESIGN--The records of the Liverpool Congenital Malformations Registry and the Royal Liverpool Children's Hospital identified 239 patients (155 male, 84 female) born with aortic valve stenosis between 1960 and 1990. Patients were traced to assess the severity of stenosis at follow up. Information on the severity at presentation and all subsequent events was obtained. RESULTS--Congenital aortic valve stenosis occurred in 5.7% of patients with congenital heart disease born in the Merseyside area. The median age at presentation was 16 months (range 0-20 years). Stenosis was mild at presentation in 145 patients, moderate in 33, severe in one and critical in 21 and 39 had a bicuspid valve without stenosis. Additional cardiac lesions were significantly more common in children presenting under one year of age and in those with critical stenosis. The median duration of follow up was 9.2 years (range 1-28 years) and seven patients were lost to follow up. 81 operations were performed in 60 patients. The reoperation rate was 28.3% after a median duration of 8.7 years (range 2.5-18 years). 15% of patients who presented with mild stenosis subsequently required operation compared with 67% of those with moderate stenosis. There were no sudden unexpected deaths and no deaths after aortic valvotomy, except in those presenting with critical stenosis. Mortality was 16.7% but patients presenting with critical aortic stenosis had a much worse prognosis. Actuarial and hazard analysis showed that the survival and absence of serious events (aortic valve surgery or balloon dilatation, endocarditis, or death) were significantly better in patients who presented with mild aortic stenosis than in those who presented with moderate aortic stenosis. 75% of patients presenting with mild stenosis had not progressed to moderate stenosis after 10 years of follow up. CONCLUSIONS--Congenital aortic valve stenosis may be progressive even when it is mild at presentation. Patients presenting with mild stenosis, however, have a significantly better prognosis than those presenting with moderate stenosis. An accurate clinical and echocardiographic assessment of the severity of aortic valve stenosis at presentation provides a good guide to prognosis into early adult life.