The authors describe two cases of penetrating ulcers of the descending thoracic aorta associated with intramural hematoma of the ascending aorta. Since neither patient had evidence of diffuse thoracic aortic atherosclerosis to limit longitudinal extent of spread, it is postulated that the intramural hematoma started at the level of the penetrating ulcer and extended into the ascending aorta in a retrograde fashion. This is a potentially lethal entity, not previously described, for which the optimal clinical management is unknown.