Occurrence of thrombus formation in the left atrium is rare without accompanying abnormalities at the mitral valve. Only a few cases of giant and pedunculated thromboses in the left atrium have been reported in the Japanese literatures. This is a case report of giant thrombus in the left atrium treated surgically. Patient was 61 years old male suffering from palpitation. Physical examination of the heart revealed increased first sound and diastolic rumbling at the apex. ECG revealed atrial fibrillation and LVH. Echocardiography and chest CT demonstrated a huge tumor like mass in the left atrium attached to the septal area. There were no abnormalities found at mitral valvular structures or motion. Catheterization studies revealed no disturbance of valvular functions. Angiographic examination demonstrated smooth surfaced tumor mass occupied left atrium. Coronary angiography revealed significant stenosis in segments 6, 7, 9, 12 and 14. Prior to surgery, patient was diagnosed myxoma of the left atrium and ischemic heart disease. Removal of the mass and CABG were carried out at a same time. Pathology revealed giant and pedunculated globular thrombus. Contributional factors possibly growing of such huge thrombus formation explained resulting from atrial fibrillation and decreased contractile force of the hypertrophied ventricle.