An example of a large, jejunal branch aneurysm is presented along with a review of 39 other cases of superior mesenteric artery branch aneurysms. current theories of etiology, including atherosclerosis, arterial dysplasia, necrotizing arteritis, and infections, are discussed. Common modes of presentation-pain, intraabdominal hemorrhage, and gastrointestinal bleeding-are reviewed. Emphasis is placed on the role of arteriography in diagnosis and management of branch aneurysms.