From 1985 to 1989, 4 neonates or infants (3 males and 1 female) underwent open valvotomy for severe aortic valve stenosis. In all patients, preoperative echocardiograms showed abnormal findings of endocardial fibroelastosis and/or poor performance of left ventricle. All patients underwent valvotomy using cardiopulmonary bypass. There was one early death from left ventricular failure. All other patients survived and the pressure gradient through aortic valve was reduced from 50-100 mmHg to 25-50 mmHg postoperatively. We conclude that infants with severe aortic valve stenosis should undergo open valvotomy before the manifestation of endocardial fibroelastosis and/or left ventricular muscle damage, and that open aortic valvotomy using cardiopulmonary bypass is most suitable method for severe aortic valve stenosis in infants.