The objectives of this study were to review the outcome of balloon dilatation of critical/severe aortic valve stenosis in patients younger than 6 months of age, with particular emphasis on subdivisions of age at intervention, and to identify factors that might influence outcome. From 1988 to 1998, 42 patients underwent dilatation. Patients were divided into three groups (group 1: 1-7 days, n = 16; group 2: 8-30 days; n = 10; group 3: 1-6 months, n = 16). Medical records and echocardiograms were reviewed retrospectively for presentation, clinical course, and left ventricular, aortic valve, and Doppler flow parameters. Median follow-up was 53 months (range, 6 months to 10 years). Of 16 group 1 patients, 11 (70%) had, respiratory distress requiring ventilator support, 12 (80%) received prostaglandin, and 5 (30%) received inotropic support. Nine (56.2%) patients died and 7 (44%) required reintervention. Of 10 group 2 patients, 4 (40%) were ventilated, 2 (20%) received prostaglandin, and 3 (30%) received inotropic support. Three (30%) patients died and 5 (50%) required reintervention. Of 16 group 3 patients, only 1 had symptoms (respiratory distress) at presentation. One (6%) patient died and 4 (15%) required reintervention. The overall actuarial survival rate at 10 years was 72% (88% at 10 years for indexed aortic annulus > 25 mm/m2. Freedom from reintervention was 70% and 21% at 5 and 10 years, respectively (80 and 33% at 5 and 10 years, respectively, for indexed aortic annulus > 25 mm/m2). The actuarial survival rates at 10 years for groups 1, 2, and 3 were 42%, 65%, and 93%, respectively. Predictors of death included young age at presentation, and multivariate analysis of left heart measures yielded an 83% positive prediction of outcome. An improved chance of survival was associated with indexed aortic valve annulus > 25 mm/m2. Patients with critical aortic stenosis who require balloon dilatation within the first month of life, but especially within the first week, have a poorer outcome than those requiring the procedure later, and this can be accounted for by a tendency toward less favorable anatomical features. Many will require repeat intervention.