Over the period January 1965-July 1992 26 spontaneous fistulas between an abdominal aortic aneurysm (AAA) and the major abdominal veins were observed and surgically treated. Twenty-two were aorto-caval, one iliaco-iliac and 3 aorto-iliac; since clinical features, pathophysiology, principles of surgical treatment and postoperative care are similar, both the conditions are considered as a single disease (aorto-caval fistula: ACF). The incidence among 373 ruptured AAA operated in emergency conditions in the same period was 6.97%, with an operative mortality rate of 34.61% compared to an overall mortality for ruptured AAA of 34.85%. All subjects were males with a mean age of 67.3 years. Twelve subjects showed shock at admission (46.1%): the mortality rate in this subgroup was 50% compared to 21.4% among the non-shocked patients. Pain was always present, oedema of one or both of the lower limbs in 9 cases (34.6%) and abdominal bruit or murmur and thrill in 16 (61.5%). One patient died at laparotomy for irreversible cardiac arrest; the 25 completed procedures consisted of endoaneurysmal repair of the fistula under venous bleeding control by digital compression and prosthetic replacement of the abdominal aorta (7 straight and 18 bifurcated grafts). Intraoperative mean blood losses exceeded 4,000 ml, but autotransfusion, available only in 12 procedures, allowed significant sparing of heterologous blood units. The mortality rate was not clearly improved by autotransfusion, but among these 12 patients shock was present in 7 instances (58.3%), compared to 5 out of 14 subjects (35.7%) operated on before autotransfusion devices were available.(ABSTRACT TRUNCATED AT 250 WORDS)