OBJECTIVE Primary renal synovial sarcoma is rare and might be misdiagnosed as another renal tumor. This study demonstrates the clinicopathologic and immunohistochemical features, differential diagnosis, and prognosis of such tumors. METHODS Histologic slides and clinical data were reviewed for 4 patients with primary renal synovial sarcoma and immunohistochemical staining was performed. Molecular analysis was performed on 2 cases to demonstrate the presence of the SYT-SSX gene fusion transcripts by reverse transcriptase polymerase chain reaction (RT-PCR). RESULTS The patients were 2 women and 2 men aged from 32 to 48 years. The tumors were 10.0-15.0 cm in diameter, grey-white and solid, and hemorrhage or necrosis was observed. Microscopically, the tumors consisted of mitotically active, monomorphic plump spindle cells with indistinct cell borders growing in short, intersecting fascicles. Hypocellular myxoid areas and a prominent hemangiopericytomatous pattern were present in all cases. The average mitotic rate was 5-8 mitoses/10 high-power fields. Hemorrhage and tumor necrosis were easily found. Scattered small cysts lined with flat, cuboidal, or hobnailed epithelia were found in 3 cases. Tumor cells are immunoreactive for Vimentin (4/4), Bcl-2 (4/4), CD99 (4/4), and CD56 (3/4), and focally for EMA (3/4) and Cytokeratin (3/4). SYT-SSX1 gene fusion was detected in the 2 cases in which RT-PCR analysis was performed. One patient had tumor metastasis to the lung 6 months after surgery and died 5 months later. Multiple metastasis to the liver occurred in one patient and the patient died 13 months after the initial surgery. The other 2 patients had tumors recur at 8 and 15 months and died at 18 and 21 months, respectively, after the initial operation. CONCLUSIONS Primary renal synovial sarcoma is rare, with poor prognosis, characterized by SYT-SSX gene fusion, and needs to be differentiated from other renal sarcomas.