A 20-year-old man was admitted to hospital because of general fatigue during exercise. He had had a heart murmur since the age of 6 years. Echocardiography showed severe mitral regurgitation (MR, IV), probably caused by an anterior leaflet cleft or tendon rupture. During surgery, a cleft measuring 9 mm in length was found in the center of the anterior leaflet of the mitral valve. The cleft was closed directly, together with annuloplasty using the bilateral Kay's method. A Cosgrove ring (32 mm) was added because the mitral valve annulus was dilated. The patient's postoperative course was uneventful and echocardiography after surgery demonstrated no MR. An isolated cleft of the anterior mitral leaflet is a rare cause of MR and in this case, direct closure of the cleft with additional annuloplasty gave a good functional result.