Between February 1988 and September 1993 balloon aortic valvuloplasty was attempted in 33 consecutive patients in the first year of life: 20 patients (61%) were younger than 1 month. Major associated anomalies such as mitral stenosis, coarctation, and hypoplastic left ventricle were found in 11 cases (33%). The balloon dilation of the aortic valve was accomplished through the right carotid cut-down approach in neonates and patients with body weight < 5 kg, through a percutaneous femoral approach in the others; the procedure was completed in all. The peak systolic gradient across the aortic valve measured at catheterization fell from 80 +/- 33 mmHg (range 25-165) before the dilation to 27 +/- 17 mmHg (range 0-65), afterwards (p < 0.0001). The left ventricular ejection fraction increased from 44% +/- 26% to 61% +/- 17%, 24-48 hours after the procedure (p < 0.0001). Aortic insufficiency developed in 17 cases, being moderate in 2, mild in 6, and trivial in 9. Seven patients (21%), all in the first month of life, died within 30 days from the valvuloplasty; major associated anomalies were present in six; the death was due to a procedure related complication in one. No mortality was observed among the patients undergoing valvuloplasty beyond the first month of life. On follow-up (6 months to 6 years) aortic restenosis occurred in 3 cases; 1 was treated by surgical valvotomy, 2 by repeat balloon valvatomy; in another 2 cases, a subvalvular aortic obstruction developed and was relieved by surgical resection. There was no late mortality. Thus, balloon valvuloplasty appears to be an effective palliation for critical aortic stenosis in infancy. Early mortality is mainly related to associated anomalies.