An aneurysms of a renal vein is very uncommon an entity and even more so when a visceral vein is affected. The venous aneurysms are generally asymptomatic and are detected either at post-mortem examination or by Echography, CT scan or MR investigation. Occasionally they become symptomatic because of rupture, thrombosis and embolism, but even in those cases they are difficult to be diagnosed and can be life threatening particularly when bleeding occurs. Exceptionally an aneurysm of a visceral vein is an unexpected intraoperative finding and is detected during an abdominal procedure undertaken for other pathology. In our experience a true aneurysm of the main trunk of the left renal vein was detected during a procedure of aorto-bifemoral by-pass graft repair for chronic aorto-iliac occlusive disease. The aneurysm was resected and the vein repaired by direct suture. Congenital weakness of the vein wall was very likely the cause as suggested by the extreme thinness and media atrophy of the aneurysm and normal appearance of the wall of renal vein and inferior vena cava. Differences between varices and aneurysms of the renal veins are discussed as well as indications for surgical treatment.