We describe a case of primary aortoenteric fistula (PAEF) in a patient with upper gastrointestinal bleeding. Primary aortoenteric fistula is a rare but clinically important cause of catastrophic gastrointestinal bleeding. The diagnosis of PAEF is difficult and sometimes not made until laparotomy. Primary aortoenteric fistula should be considered in any patient known to have an abdominal aortic aneurysm who presents with upper gastrointestinal bleeding when no identifiable source of bleeding is found on upper endoscopy. Computed tomography may confirm the diagnosis and emergency exploratory laparotomy should be done as soon as the diagnosis is considered clinically.