A cleft in the anterior mitral valve leaflet is commonly associated with atrioventricular septal defects, but may occur as an isolated phenomenon. The cleft mitral valve can cause regurgitation, and surgical treatment of the cleft leaflet should conserve the existing valve if possible. But if satisfactory hemodynamic security cannot be obtained with a reconstructive procedure, the valve should be replaced. We present one case of an isolated cleft in the anterior mitral valve leaflet. The patient showed marked mitral regurgitation in the preoperative left ventriculogram. We managed him successfully by interrupted suture repair of the cleft, and he was discharged uneventfully.