Venous aneurysm of the azygos arch is a very rare cause of mediastinal mass and is usually an incidental finding on chest radiography. Nowadays the diagnosis is made by non-invasive tests such as thoracic CT scanning and/or magnetic resonance imaging. The case is described of an asymptomatic woman in whom a mediastinal mass due to an azygos vein aneurysm was diagnosed by non-invasive procedures, the aetiology of which, in all probability, was idiopathic.