We describe a 50-year-old man with a secondary aortoduodenal fistula who presented with high fever and right leg pain one year after undergoing an aortoiliac bypass with a polyester graft. Gangrene had developed in the right ankle, and contrast-enhanced computed tomography (CT) revealed that the graft had penetrated the third duodenal segment and obstructed the right graft limb. Esophagogastroduodenoscopy confirmed that the graft had perforated the duodenum. A preoperative diagnosis of aortoenteric fistula can be very difficult. In spite of the lack of gastrointestinal bleeding in this case, we directly diagnosed secondary aortoduodenal fistula preoperatively using computed tomography and esophagogastroduodenoscopy. Secondary aortoenteric fistulae should be suspected when a patient with an aortic prosthesis shows symptoms in the lower limb.