A 57-year-old female with hypotension and consciousness disturbance was referred to our institute. Echocardiography and plain CT revealed cardiac tamponade, but no apparent evidence of aortic dissection was obtained. With sudden onset of hypotension, she was immediately operated without information of aortography, enhanced CT, and MRI. Approximately 200 g of pericardial hemorrhage and slight subadventitial hematoma was grossly found and surface ultrasonography revealed the mild localized intramural hematoma (crescent-shaped translucent layer of at most 5 mm in width) at the proximal ascending aorta. Because neither false lumen with blood flow nor intimal flap was found, pericardial drainage without aortic replacement was performed. However, DeBakey type II of aortic dissection with wide false lumen was found with CT and MRI within three weeks, and re-operation was carried out. The dissection ranged from the ostium of right coronary artery to the beginning of innominate artery, but did not involve the coronary arteries and major branches of aortic arch. The surface ultrasonography and transesophageal echocardiography, as well as preoperative CT and MRI, demonstrated that false lumen was thrombosed except at the anterior aspect of mid-portion of ascending aorta. The ascending aorta was replaced. A careful follow-up is mandatory for even a localized intramural hematoma, which can progress into aortic dissection in a short period.