We report a case of a solitary iliac aneurysm-fistula of the sigmoid colon. A 68-year-old male was diagnosed as having diverticulum of the sigmoid colon by barium enema at a near-by hospital with a major complaint of melena. He continued to have massive melena although he received sigmoid colectomy. His condition eventually deteriorated into shock and he was transferred to our department. Angiographic findings showed a left common iliac aneurysm. Under the diagnosis of a rupture of a sigmoid colon, emergency operation was performed including aneurysmectomy and bypass formation between the femoral and femoral artery as an extraanatomical bypass. The patient developed multipul organ failure following the sepsis and died 8 days postoperatively. An aneurysm-intestinal fistula is a complication of an aneurysm. The problem of this disease is the difficulty in making a definite diagnosis with high mortality rate. We should consider the possibility of an aneurysm-intestinal fistula for the patient with gastrointestinal bleeding of the unknown origin.