This communication describes a new surgical procedure of enlarging the narrow aortic valve ring by extending the aortic incision through the fibrous origin of the aortic leaflet of the mitral valve into this leaflet. A fusiform patch is sutured to the V-shaped defect in the aortic leaflet of the mitral valve and in the aortic anulus. This procedure permits the replacement of the aortic valve by a suitable prosthesis. Between June of 1976 and February of 1978, eight patients underwent this surgical procedure. At the time of operation the patients were between 8 and 50 years old. The estimated enlargement of the aortic root ranged from 10 to 25 mm. The operative technique is described, peculiarities of this method are discussed, and the results are reported. Six to 27 months following operation, the clinical condition of six patients is good. Four patients show no impairment of mitral valve function. In one case, preoperatively diagnosed mitral incompetence persists. In another patient the pericardial patch broke from the aortic leaflet of the mitral valve, so that the valve had to be replaced on the fourth postoperative day. One patient died of myocardial necrosis because of insufficient myocardial protection during operation. One child with acute aortic insufficiency caused by staphylococcal endocarditis and congestive heart failure died of septicemia 3 months postoperatively. Mitral incompetence was not detectable in this child.