Two hundred and eighteen patients with congenital valvar aortic stenosis aged between 1 and 25 years at presentation were followed for one to 26 years (average 8.7 years). Assessment of severity of aortic stenosis was essentially based on clinical findings including symptoms, the character of the pulse, and the behaviour of the second heart sound, management being further influenced by the degree of left ventricular hypertrophy shown in the electrocardiogram. There was reasonable correlation between clinical and haemodynamic assessment and all patients with a gradient between left ventricle and aorta (LV-Ao) greater than 50 mmHg (6.7 kPa) were judged to have moderate or severe aortic stenosis on physical signs. Fifty-five per cent of those judged mild on presentation still had a mild lesion 18 years later; 42 per cent of those judged moderate on presentation still had a moderate lesion 15 years later. Forty-four patients had an operation and of the 30 patients who underwent aortic valvotomy, only 40 per cent still had a satisfactory result 13 years later. There were three preoperative and seven postoperative deaths but in only two patients was death directly related to severe aortic stenosis and both of these patients had been lost to follow-up. It is concluded that indications for operation are aortic stenosis which is moderate or severe on physical findings, together with restrictive symptoms or ST and T wave changes on the electrocardiogram. Using these criteria, the risk of sudden death is minimal and irreversible myocardial damage unlikely. We do not recommend either routine cardiac catheterisation or routine operation at any arbitrary LV-Ao gradient.