5 cases of aorto-iliac aneurysms which fistulised into the inferior vena cava are described. They illustrate the problems encountered in this condition. - METHODS although a classical diagnosis, it is only recognised in 27 p. 100 of cases because the clinical presentation can be very variable. A pulsating, expansive, abdominal mass very suggestive of an aneurysm of the abdominal aorta should alert to the possibility of a fistula within the inferior vena cava when the patient has unexplained cardiac failure, shock, lower limb deep vein thrombosis, urinary or psychological problems. In the absence of a continuous abdominal murmur, the diagnosis may be confirmed by complementary investigations; the investigations of choice are non-invasive. Abdominal ultrasonography showing an aneurysm of the abdominal aorta associated with dilatation of the inferior vena cava is very suggestive of the diagnosis. - The only treatment is surgery. The operative risk depends on the presence of absence of associated retroperitoneal rupture of the aneurysm, the respective mortalities being 83 p. 100 and 15 p. 100. - The principal causes of death are pulmonary embolism and renal failure which necessitate special prophylactic measures (surgical and anaesthetic).