A patient with a known infrarenal atherosclerotic aneurysm measuring 8 cm presented with sudden dyspnoe, lower back pain and enormous venous congestion of the lower extremities. Spiral CT rendered precise diagnosis and localisation of a large and acute spontaneous aorto-caval fistula and made early surgical intervention possible. Abdominal aorto-caval fistulas are rare complications and can occur after surgery, trauma or spontaneously in an atherosclerotic or inflammatory aneurysm. Spiral CT enables a rapid and precise diagnosis facilitating the planning of the therapeutic approach to this complication.