OBJECTIVE--To assess the results of balloon dilatation of the aortic valve as first line treatment for infants presenting in the first two months of life with cardiac failure caused by severe aortic stenosis. DESIGN--A prospective study of all such infants presenting to a single centre over a three year period. PATIENTS--Of 14 consecutive infants aged < 2 months presenting with heart failure due to severe stenosis of the aortic valve, 12 underwent balloon dilatation of the valve. Two infants with hypoplastic left heart syndrome were excluded. Six patients had other significant cardiac malformations, nine had endocardial fibroelastosis. Four infants were moribund at presentation. INTERVENTIONS--Balloon dilatation was performed through the femoral artery under general anaesthetic, with heparin and antibiotic prophylaxis. Maximum inflated balloon size was selected as equal to or slightly less than the diameter of the aortic valve measured echocardiographically. In two infants balloon dilatation of coexistent severe coarctation was performed at the same time. RESULTS--Dilatation was achieved in all cases. Early survival rate (up to one month) was 75%; 50% survive up to three years. The two deaths occurring < 24 hours after dilatation probably related to the infants' poor condition at presentation and the presence of significant left ventricular hypoplasia. The other early death was due to severe mitral regurgitation from papillary muscle dysfunction. All later deaths were related to associated malformations. The difference in left ventricular size (diameter and length) measured echocardiographically at presentation between survivors and those that died is significant (p < or = 0.002). 83% of deaths occurred in infants with associated malformations. Cusp perforation occurred in one infant, which may have contributed to this patient's death. Left ventricular perforation and cardiac tamponade were treated successfully by prompt pericardiocentesis in another. CONCLUSIONS--Balloon dilatation of the aortic valve is an acceptable first line treatment for neonates with severe symptomatic stenosis of the aortic valve. Outcome depends principally on left ventricular size and on the presence, severity, and timing of treatment of associated malformations. Severe endocardial fibroelastosis was not a risk factor for early death but may have implications for long term survival. The survival rate for this high risk group might be improved by a stratified approach where patients with adequate left ventricular size have balloon dilatation whereas those with severe ventricular hypoplasia are managed like patients with the hypoplastic left heart syndrome.