We report the successful treatment of a rare case of chronic expanding intrapericardial hematoma that had slowly developed into a large mass after coronary artery bypass surgery. An 85-year-old man with a history of coronary artery bypass surgery presented with dyspnea on exertion and leg edema in 2006. Chest roentgenograph demonstrated right pleural effusion and severe pulmonary edema. An echocardiographic study demonstrated a mass located posterior to the left ventricle that severely compressed the left ventricle toward the ventricular septum. Surgical resection of the mass was planned to release the symptoms and to confirm the diagnosis of the mass. The mass was completely resected through a left thoracotomy, and the histological findings confirmed the diagnosis of a chronic expanding intrapericardial hematoma. The patient's postoperative course was uneventful, and his symptoms improved markedly. There has been no sign of recurrence 1 year after the operation.