Five cases of primary synovial sarcoma of the pleura are presented with a discussion of differentiation from other biphasic malignant neoplasms, most notably malignant mesothelioma. The cases have clinical, histologic, and immunohistochemical features consistent with synovial sarcoma of soft tissue. The average age at initial presentation of the reported patients was 25 years with an approximate range of 9 to 50 years. A large pleural-based intrathoracic mass was identified in each case. Histologic analysis showed a biphasic (mixed) pattern composed of epithelial and spindle cells. The epithelial cells showed cleft-like to tubulopapillary growth with focal intraluminal periodic acid Schiff's (PAS)-positive, diastase-resistant secretions identified in four of the five cases. The spindle cell component was composed predominantly of densely packed, elongated, fusiform cells. Immunohistochemical staining showed positivity with antibodies against cytokeratin, BER.EP4, epithelial membrane antigen, and vimentin in all cases. The patients seem to have an aggressive course, with four deaths reported within 3 years from initial surgery. These cases represent the first reported cases of primary synovial sarcoma of the pleura and lend further credence to the theory that synovial sarcomas are derived from immature mesenchymal elements, not from synovium.