A biphasic synovial sarcoma occurring in the wall of a pathologically thickened femoral vein was diagnosed in a 34-year-old woman. This sarcoma was resected together with removal of a thrombus. A local recurrence was seen 5 years later, but the patient is alive and well 11 years after the first operation. Immunohistochemical evaluation revealed cytokeratin and epithelial membrane antigen in a portion of the tumor cells, as are typically seen in synovial sarcoma. This case shows that so-called synovial sarcoma may arise in places remote from any joints and that its origin may not be a synovial or perisynovial tissue in all cases. More likely, synovial sarcomas develop from transformed mesenchymal cells, some of which have acquired the epithelial phenotype.