Sixty four children with isolated congenital aortic stenosis (39 valvular, 16 fixed subvalvular, 4 supravalvular and 5 multiple) were operated at a mean age of 11,5 years. Valve repair was possible in all but three patients who had to undergo valvular replacement. Myotomy was associated in 18 cases (28 p. 100). The mean systolic pressure gradient was 79,9 mmHg (+/- 17,8); there was associated aortic regurgitation in 21 patients but this was minimal except in one case. Twenty children (31 p. 100) had symptoms on effort and the basal ECG showed ST-T wave changes in the left precordial leads in 30 cases (47 p. 100). Several preoperative exercise ECGs were performed in 29 patients without ST-T changes on the resting ECG. The exercise ECG was positive in 15 patients, providing one of the arguments for surgery; a poor blood pressure response to exercise was observed in 12 patients with a negative test. Out of the 28 patients with a positive preoperative exercise ECG, 7 (25 p. 100) went on having a positive result after surgery (p less than 0,05). The maximal heart rate was not significantly higher after surgery but the total work was significantly greater (p less than 0,01) and the increase in systolic blood pressure was even more significant (p less than 0,001). Out of 14 patients undergoing repeat catheterisation for a continuing positive exercise ECG or for ST-T wave changes on the resting ECG, there were 6 residual severe stenoses, 3 severe aortic regurgitations, 3 hypertrophic cardiomyopathies which were obstructive in 2 cases. The exercise ECG is a means of appreciating the consequences of the stenosis which are the cause of the complications (myocardial ischemia and poor blood pressure adaptation). This justifies its use in assessing the surgical indications and for the follow-up of the surgical result. A persistantly positive exercise ECG and continuing ST-T wave changes on the resting ECG are signs of a poor surgical result and hemodynamic revaluation should be considered; besides severe postoperative aortic regurgitation, residual or recurrent stenosis and, above all, asymmetric septal hypertrophy, obstructive or not, are the main causes of poor postoperative results.