The optimal treatment of critical aortic stenosis in the neonate and infant remains controversial. We compared transventricular dilation using normothermic cardiopulmonary bypass and percutaneous balloon aortic valvuloplasty with respect to early and late survival, relief of aortic stenosis, degree of aortic insufficiency, left ventricular function, and freedom from reintervention. Between July 1987 and July 1993, 30 neonates and infants underwent transventricular dilation or balloon aortic valvuloplasty for critical aortic stenosis. The patients in the transventricular dilation group (n = 21) ranged in age from 1 to 59 days (mean age 18.0 days +/- 19.1 days) and the balloon aortic valvuloplasty group (n = 9) from 1 to 31 days (mean age 10.0 days +/- 9.0 days). There were no significant differences in weight, body surface area, or aortic anulus diameter between the two groups (p = 1.0). Associated cardiovascular anomalies were more common in the transventricular dilation group (48%) than in the balloon aortic valvuloplasty group (11%). After intervention, the degree of residual aortic stenosis and insufficiency was equivalent in the two groups as assessed by postprocedural Doppler echocardiography. Ejection fraction improved within both groups (transventricular dilation 39% +/- 20.2% versus 47% +/- 22.0%; balloon aortic valvuloplasty 51% +/- 16.1% versus 62% +/- 8.4%), and there was no significant difference between groups. The left ventricular mass/volume ratio increased within both groups (p < 0.05) but with no significant difference between groups (transventricular dilation 1.4 +/- 0.5 gm/ml versus 1.8 +/- 0.6 gm/ml; balloon aortic valvuloplasty 1.1 +/- 0.6 gm/ml versus 1.7 +/- 0.4 gm/ml). Early mortality in the transventricular dilation group was 9.5% and in the balloon aortic valvuloplasty group, 11.1%. There was one late death in the transventricular dilation group. Four patients from the transventricular dilation group (19%) and two patients from the balloon aortic valvuloplasty group (22%) required reintervention for further relief of aortic stenosis. We conclude that both transventricular dilation and balloon aortic valvuloplasty provide adequate and equivalent relief of critical aortic stenosis. The treatment strategy adopted should depend on other factors, including associated cardiovascular anomalies, vascular access, preoperative condition, and the technical expertise available at each institution.