Superior mesenteric branch aneurysms are rare and usually become symptomatic at the time of rupture. Pain, gastrointestinal blood loss and intra-abdominal hemorrhage draw attention to the presence of aneurysms in 70% of the reported cases. We report on a 64-year-old male patient who had an emergent laparotomy for acute abdominal pain at a local hospital in Changhua in March of 1988. The operative finding was an unresectable mesentric mass, and the pathologic finding of the biopsy was a chronic abscess. Because of two episodes of tarry stools after the operation, the patient was referred to the National Taiwan University Hospital for further evaluation of the intra-abdominal mass. After admission in April of 1990, abdominal sonogram and CT examinations demonstrated the presence of a multilobulated mass which was suspected to be an aneurysm. Selective superior mesenteric arteriography confirmed this diagnosis and showed that the aneurysm arose from the origin of the ileocolic branch. At surgery, the aneurysm was found to have a fistula tract communicating with the terminal ileum. The aneurysm and the associated segment of the terminal ileum were successfully removed. We herein report this unusual case.