Aneurysm of the azygos vein is a rare cause of mediastinal mass that is normally detected radiologically in asymptomatic patients. In some cases a diagnosis is made by noninvasive methods, such as computed tomography and magnetic resonance. However, in the present case imaging revealed what appeared to be a solid mass. When the mass had been excised by thoracotomy, the histopathologic diagnosis was aneurysm of the azygos vein.