Clinically, penetrating atheromatous ulceration of the aortic wall may be confused with either symptomatic thoracic aneurysm or classic spontaneous aortic dissection. Aortography and computed tomographic (CT) scanning of the thoracic aorta provide specific diagnostic information which permits one to distinguish this lesion from atherosclerotic aneurysm and classic dissection. Hallmarks of findings on aortography and CT scan include the presence of the ulcer and an intramural hematoma. Since the findings may be disarmingly subtle, the potentially progressive and serious nature of this condition may remain unappreciated. Recognition of the penetrating atheromatous ulcer and distinguishing it from aortic dissection arising just distal to the origin of the left subclavian artery is mandatory. Resection of only a conservative segment of the proximal descending aorta suffices for classic dissection in the upper descending thoracic aorta, but the penetrating aortic ulcer requires graft replacement in the area of the ulcer and intramural hematoma.