A 62-year-old woman was admitted to our hospital due to congestive heart failure. On the chest X-ray film, cardiomegaly and prominent pulmonary congestion were revealed. She became well with bed rest and medication of digitalis and diuretics. Echocardiogram established the diagnosis of severe mitral regurgitation. In addition, cineangiography of the left ventricule showed cystic left ventricular aneurysm which originated from below the posterior mitral annulus. Operation was performed on July 16, 1990. Under cardiopulmonary bypass, we reflected the apex of the LV upwards, but failed to identify the aneurysm. Then, left atrium was opened through Dubost incision. As regards mitral valve structures, there were no abnormal findings except moderate enlargement of it's orifice. After resection of the mitral valve, orifice of the aneurysm (0.5 X 2.0 cm) was recognized, which was closed with the buttressed sutures. Afterwards, MVR was performed with 25 mm Medtronic Hall valve. The patient had a smooth postoperative course without complication.