Accumulation of free fluid in the peritoneal cavity is called ascites. The first step in identifying its etiology is to determine the serum-ascites albumin gradient (SAAG). According to this parameter, a high SAAG is regarded as a gradient greater than 1.1 g/dL. This condition includes some differential diagnoses such as liver cirrhosis, Budd-Chiari syndrome, heart failure, and idiopathic portal fibrosis.

In the present article, we introduce a young man with abdominal distention due to a high SAAG ascites. Further evaluation revealed Budd-Chiari syndrome caused by a mediastinal mass. Diagnosis based on pathologic study was mediastinal fibrosarcoma.

Mediastinal fibrosarcomas, though rare, should be considered in the differential diagnosis of mediastinal masses.