A case of azygos vein aneurysm is described. It is a very rare condition, this being the tenth reported case. The patient presented with superior vena cava obstruction and underwent successful surgical cure. This congenital malformation is practically always latent. The aneurysm is usually diagnosed after the discovery of a mediastinal mass on routine chest X ray. Although its variations in size with position and respiration are characteristic, angiography would seem to be essential to confirm its aneurysmal nature and the anatomical relationships. Surgery is usually simple and should be proposed even to asymptomatic patients because of the risk of thromboembolism.