Operative balloon dilatation of the aortic valve was performed in seven neonates with critical stenosis of the aortic valve. The procedure was followed by the development of severe aortic regurgitation in four patients. Necropsy was performed in three and revealed partial detachment of the right coronary cusp of the aortic valve. Damage to the valve leaflet caused by balloon dilatation was probably the result of using a balloon with a diameter that was too large in relation to the aortic valve ring diameter and of shearing forces created in the aortic wall by the contracting ventricle. The diameter of the inflated balloon should not be larger than the diameter of the aortic valve ring.