The authors report two cases of arteriovenous fistula due to spontaneous rupture of an aortic or iliac aneurysm into the iliocaval venous axis. This is a rare complication of atheromatous aneurysm (less than 4% of ruptured aneurysms), often difficult to diagnose as the clinical presentation may be obscure. Although aortography is the reference diagnostic investigation, color Doppler ultrasonography enabled visualisation of the arteriovenous communication and provided an accurate diagnosis in one recent case. Treatment of these aortocaval fistulae is always surgical. The prognosis and immediate operative mortality depend mainly on the presence of an associated retroperitoneal rupture.