During a period of 13 years 11 patients were operated on because of a spontaneous aortocaval fistula caused by a ruptured abdominal aortic aneurysm. The classic diagnostic signs of an aortocaval fistula (pulsatile abdominal mass with bruit and high output heart failure) were present in approximately half of the patients, whereas hematuria was a constant finding in all patients. Six patients had macrohematuria, and five had microhematuria. Seven patients (64% survived, and four had postoperative complications: 1 ileus, 2 postoperative pneumonias, 2 deep venous thrombosis, 1 postoperative hemorrhage. The mean operative blood loss was 7 L. After operation the average follow-up time was 4 years. In four patients who died the perioperative (within 30 days) causes of death were renal failure, a bleeding duodenal carcinoma, myocardial infarction, and operative bleeding. It is concluded that hematuria is a more frequent finding than earlier assumed among patients whose abdominal aortic aneurysm has ruptured into the vena cava. The presence of hematuria in a patient suffering from an abdominal aortic aneurysm is an indication for aortography to rule out an aortocaval fistula.