A 22-year-old man was referred to our medical center with an impending rupture of an aneurysm of the descending thoracic aorta. Blood pressure was 180/110mmHg in the right arm but 110/60mmHg in the right foot. The diagnosis was confirmed by chest Xray, enchanced computed tomography and aortogram. Five days later, the chest Xray showed massive effusion in the left pleural cavity. Surgery was immediately performed via a left thoracotomy. Five hundred ml of bloody fluid was found in the pleural cavity but the site of bleeding could not be identified. The aneurysm was 7×10cm in size. Under percutaneous cardio pulmonary support, the aneurysm was replaced by a 22mm Gel-Seal Dacron vascular graft. The intima and the media of the aneurysm were lacerated longitudinally at the region receiving jet flow from isthmus, There was blood coagula between the media and adventitia. During replacement, activated clotting time was maintained at 200∼300sec. As a result, bleeding was limited to 200ml. The postoperative course was uneventful with little difference in pressure between the right arm and right foot.