Ten children in the age range of 3 to 17 years with moderate to severe aortic valve stenosis (gradients of 55 to 109 mm Hg) underwent cardiac operations. At the time of the operation, during bypass, balloon dilation of the stenotic valve was performed and the results were visually assessed by the surgeon. Of the 10 consecutive cases, only three showed dilation results that were comparable to what seems optimal from a surgical point of view. The adverse effects in the remaining seven patients and the corrective measures taken were as follows: (1) too extensive a rupture requiring stabilizing sutures in one patient; (2) too short a rupture, requiring additional commissurotomy in three patients; (3) rupture into the valve leaflet, requiring valve suture and corrective comissurotomy in another three patients. This last, rather serious complication occurred in patients having functionally bicuspid valves with slightly thickened free valve edges, whereas valves with severely thickened edges ruptured in the commissure line but often to an insufficient degree. Because of the high incidence of suboptimal separation of the stenotic aortic valves with balloon dilation, we recommend that further evaluation of long-term results and identification of unsuitable cases should precede widespread use of the technique.