Acute spontaneous arterio-venous fistula complicating atherosclerotic abdominal aortic aneurysm (AAA) is rare. This life-threatening setting is observed in 1-2% of all AAAs and 2-4% of ruptured of AAAs. The triad of abdominal or lower back pain, pulsatile abdominal mass, and continual abdominal machinery-like bruit is seen only in half of cases. Currently, CT angiography is a noninvasive technique which enables a rapid and exact preoperative diagnosis. The authors describe three cases of aortoiliac aneurysm complicated by an acute arteriovenous fistula which were diagnosed using spiral CT.