Twenty-two spontaneous aortocaval fistulas between an abdominal aortic aneurysm and the inferior vena cava were surgically treated in 27 years at one surgical unit. The incidence was 5.9% of ruptured abdominal aneurysms; the operative mortality rate of 36.4% compared with an overall mortality rate for ruptured abdominal aortic aneurysms of 34.9%. Among 10 subjects in shock on admission the mortality rate was 50% compared with 25% in non-shocked patients. Of the 22 patients one died at laparotomy from irreversible cardiac arrest; in the other 21 the procedure consisted of endoaneurysmal repair of the fistula which involved replacement of the aneurysm by a Dacron prosthesis after control of venous bleeding was achieved. No occurrence of paradoxical pulmonary embolism was reported. Multiple organ failure caused death in six cases; of these, four died as a result of acute renal failure. Severe preoperative anuric shock was recorded in five instances, with a mortality rate of 80%, compared to 25% for non-shocked subjects. Mortality was not improved by intraoperative autotransfusion; however, the incidence of severe shock was 55.5% in those patients treated by autotransfusion, compared with 38.5% in the standard blood replacement group.