A 74-year-old man who had received graft replacement of ruptured abdominal aortic aneurysm 6 years previously was admitted to our hospital because of hematemesis. Gastroduodenoscopy revealed no bleeding site in the stomach or the first and second portions of the duodenum. Preoperative CT scan demonstrated an adhesion of the aorta-duodenum at the proximal anastomosis of the prosthetic graft. Preoperative angiography demonstrated no leak of contrast material at the proximal anastomosis of the prosthetic graft. Therefore, we performed an emergency operation under a diagnosis of an aortoduodenal fistula. Operative reconstruction was performed with in situ grafting using a new prosthetic graft, and the greater omentum was used to fill defects surrounding the anastomotic site. We report a case of surgical treatment for an anastomotic aneurysm associated with a graft-duodenal fistula after abdominal aortic aneurysm repair.