The diagnostic features and operative results of six patients with spontaneous aorto-caval fistula associated with abdominal aortic aneurysm were analyzed. Abdominal pain, pulsatile abdominal mass and haematuria were constant preoperative findings in all patients. Radiological signs of congestive heart failure of various degrees were present in five, abdominal bruit in four and preoperative renal failure in three patients. As preoperative diagnostic examinations i.v. pyelography was done in two patients and ultrasound scanning and angiography of the abdominal aorta in a further two patients. In one ultrasound scanning a dilated inferior vena cava and hepatic veins were seen as an indirect sign of ACF, while in both angiograms the ACF was seen. In these two cases the diagnosis of ACF was made preoperatively, while in four other cases the diagnosis was made during the operation. Three patients survived the operation and were still alive after eight months, four years and six years respectively. Postoperative complications developed in two patients: postoperative ileus in one and deep venous thrombosis and pneumonia in another. Because of its rarity aorto-caval fistula is difficult to diagnose. The presence of haematuria in a patient suffering from abdominal aortic aneurysm should strongly suggest the diagnosis of an aorto-caval fistula.