Severe supravalvular aortic stenosis was discovered at reoperation in 3 children with recurrent stenosis of the left ventricular outflow tract. The lesion consisted of gross thickening of the sinus ridge, most marked at the site of the previous aortotomy. The thickening had shortened the free edge of one or more leaflets and had distorted adjacent commissures. Preoperative angiography had indicated valve stenosis, but little commissural fusion was found at operation and the outflow tract obstruction was relieved by inserting a prosthetic patch in the ascending aorta. Recognition of this iatrogenic pathology is important to avoid supravalvular stenosis after aortic valvotomy and unnecessary valve replacement in children requiring reoperation for recurrent left ventricular outflow tract obstruction.