This study reports five cases of primary pleural monophasic synovial sarcomas and assesses the role of the SYT-SSX fusion transcript in the differential diagnosis. Patients had a mean age of 47 years with no gender predilection. Chest pain and pleural-based masses with effusions characterized the clinical presentations. Each patient underwent a complete surgical resection of the mass. The mean follow-up was 9 months, available in four patients. They were all alive, with no evidence of disease. Histologically, neoplasms were composed of densely packed fusiform cells focally alternating with less cellular areas. No epithelial differentiation was identified at the hematoxylin and eosin level. Keratin and epithelial membrane antigen reactivity was focal and present in four and two tumors, respectively. There was no immunoreactivity for CD34. RT-PCR studies for the presence of a SYT-SSX1 or SYT-SSX2 fusion transcript were positive in every tumor. In comparison, 10 localized fibrous tumors were immunohistochemically negative for keratin and epithelial membrane antigen and positive for CD34. A SYT-SSX fusion transcript was not identified in any of five localized fibrous tumors tested. Identification of the synovial sarcoma-specific chimeric transcript (SYT-SSX1 or SYT-SSX2), in conjunction with immunoperoxidase studies, can be extremely helpful in identifying cases of pleural monophasic synovial sarcoma.