OBJECTIVE--To evaluate the immediate and long-term results of transcatheter balloon dilatation of the aortic valve for restenosis after previous surgical valvotomy. DESIGN--Prospective follow up by clinical examination and cross sectional Doppler echocardiography of all patients fulfilling the above criteria. SETTING--Tertiary paediatric cardiology referral centre. PATIENTS AND METHODS--22 patients (18 male, 4 female)--median (range) age 157.5 (12-254) months--underwent 25 balloon dilatation procedures at a median of 72 (8-155) months after surgery. The median age at surgical valvotomy was 82.5 (0.5-230) months and the systolic gradient across the aortic valve immediately after surgery was 31 (0-49) mm Hg. The indication for dilatation was a Doppler derived peak instantaneous gradient of > 60 mm Hg with grade 2 or less aortic regurgitation. A single balloon was used, and the median balloon to annulus ratio was 1 (0.9-1). RESULTS--After dilatation the catheter pullback gradient decreased acutely from 55 (35-75) to 30 (0-75) mm Hg (p < 0.01) and the Doppler gradient from 74 (52-92) to 40.5 (30-96) mm Hg (p < 0.01). In three patients who underwent a second dilatation of the aortic valve eight months after the first procedure, the pullback gradient decreased from 50 (50-60) to 15 (15-16) mm Hg. Aortic regurgitation grade increased from 1 (0-2) to 2 (1-3); only one patient had grade 3 regurgitation. Over a median follow up of 33 (2-67) months seven patients had aortic valve replacement for recurrent stenosis (six patients) or severe regurgitation (one patient with grade 3 regurgitation after dilatation, who had partial detachment of one of the valve leaflets). There was no significant difference for the pullback gradient (median of 19 v 32.5 mm Hg), Doppler gradient 24 hours after dilatation (33.5 v 50.5 mm Hg; p = 0.03), or the duration of follow up (27.5 v 18 months) between the 12 patients who did not require further dilatation or surgery and the 10 patients who did. CONCLUSIONS--Balloon dilatation of the aortic valve is a safe and feasible option for palliation of restenosis after surgical valvotomy for congenital aortic valve stenosis. In many patients, however, stenosis progressed and a further intervention was required.