A 5-year-old-boy presented with a right pleural effusion and fever. A tumorous mass was located between the right lung and the liver. The boy underwent a right thoracotomy with excision of the tumor and the adherent parts of the right hemidiaphragm. Histology and immunohistochemistry showed an inflammatory pseudotumor of the diaphragm. This is the first reported case of an inflammatory pseudotumor in this location. A brief review of the heterogeneous theories about the pathogenesis and the different therapeutic regimens for this rare neoplastic entity are discussed.