Aortocaval fistula caused by rupture of an abdominal aneurysm was diagnosed before surgery and treated successfully using cardiopulmonary bypass, which effectively controlled the massive bleeding. Rupture of the false lumen of a dissecting aneurysm limited to the abdominal aorta was responsible for the aortocaval fistula. The defect of the inferior vena cava was closed with a running suture and the aneurysm replaced with a collagen-coated Y graft.