Synovial sarcomas sometimes have areas where cells are more pleomorphic and mitotically active than usual, and which can display necrosis. These appearances predominate in approximately 20% of cases. These tumors, termedpoorly differentiated synovial sarcomas,are aggressive; the clinical course can be prolonged. It is important to distinguish poorly differentiated synovial sarcomas from other soft tissue sarcomas. This may be difficult, especially in a small biopsy specimen, because the neoplastic cells can be rounded, polygonal, rhabdoid, or spindled, and thus resemble those of many other tumors. Because the immunohistochemical profiles of poorly differentiated synovial sarcomas and morphologically similar tumors can overlap, a panel of antibodies and, if necessary, electron microscopy should be used. Demonstration of the specific translocation t(x;18) (p11.2;q11.2) confirms the diagnosis.