Presentation of aortic dissection (AD) typically includes chest and/or abdominal pain. Gastrointestinal (GI) symptoms other than abdominal pain are uncommon. Two patients with AD are described in whom the dominant presenting symptom was GI hemorrhage. Mesenteric infarction complicated acute Type I dissection in one patient whose clinicalo manifestations simulated ulcerative colitis. In the other patient an old, small asymptomatic Tupe III AD resulted in a false aneurysm in the retroperitoneum which ruptured into the duodenum. In the latter case an antemortem diagnosis was not made as angiography was limited to the visceral arteries and the abdominal aorta without appreciation of the significance of a focal compression of the abdominal aorta. The possibility of AD thus should also, be considered in the evaluation of a patient with acute GI bleeding. © 1986 Springer-Verlag New York Inc.