A giant venous varix of the azygos arch is a very rare cause of a mediastinal mass. The usual diagnosis of a mediastinal mass by mediastinoscopy or percutaneous fine-needle aspiration or biopsy is very hazardous if there is a venous varix. Noninvasive thoracic CT scanning is a safe and better choice for diagnosis. We describe the case of a woman with a posterior mediastinal mass caused by a giant azygos vein varix. Thoracic CT documented the diagnosis. The etiology of the azygos varix was portal hypertension secondary to liver cirrhosis.