Unusual aspects of aortovenous fistulas associated with ruptured abdominal aortic aneurysms. Three unusual cases of an abdominal aortic aneurysm spontaneously rupturing into the retroperitoneum and an adherent posterior vein are reported. No patient demonstrated signs or symptoms of an aortovenous fistula before surgery. Emergent surgery prevented extensive preoperative diagnostic testing. The three abdominal aortic aneurysms were very large and averaged 13 cm in diameter. One fistula involved an inflammatory aneurysm, which is the fifth such case reported. The aortic fistulas were to the inferior vena cava (158 other cases reported in the English-language literature), a left renal vein (16 other cases reported), and an iliac vein (7 other cases reported). Routine use of the cell-saver, oversewing of the fistula from within the aneurysm, and a heightened awareness that this unusual complication is more common in the presence of a very large, ruptured abdominal aortic aneurysm should improve survival. 