We report the clinicopathologic and immunohistochemical features of 16 cases of genetically confirmed primary synovial sarcoma of the kidney. The cases occurred in 9 men and 7 women ranging in age from 17 to 78 years (mean, 46 y). The tumors were grossly large, solid, and variably cystic (2.2 to 19.0 cm; mean 8.6 cm). Microscopically, all tumors were of the monophasic type and diffusely immunoreactive for TLE1 and BCL-2. Focal pankeratin positivity was found in just under half. Ten cases carried an SS18-SSX2 fusion transcript, and 5 cases showed an SS18-SSX1 transcript by reverse transcription polymerase chain reaction. The remaining case demonstrated SS18 rearrangement by fluorescence in situ hybridization. Clinical follow-up information was available for 12 patients (range, 1 to 77 mo; mean, 32.5 mo). Fourteen patients underwent radical nephrectomy, and 3 patients had lung metastases at presentation. Six patients died of disease within 1 to 58 months (mean, 31 mo) of their diagnosis. Five patients were alive without evidence of disease 12 to 77 months (mean, 39 mos) after surgery. A single patient was alive with metastases to the spine 11 months after surgery. We conclude that renal synovial sarcoma is an aggressive tumor, with adverse patient outcome in >50% of cases. Synovial sarcoma must be distinguished from morphologically similar lesions of the kidney.