We present a case report of a 17-year-old patient with a large renal mass that was detected on a computed tomography scan during investigation for secondary hypertension. Radical nephrectomy was performed and the morphologic and immunocytochemical findings were compatible with a diagnosis of monophasic synovial sarcoma of the kidney. A cytogenetic search for t(X;18) translocation was performed, which was negative. The patient underwent an ifosfamide-based chemotherapy regimen. During follow up, a positron emission tomography scan showed increased 18F-fluorodeoxyglucose metabolism at the right femur. Although cancer cells were expected in the biopsy specimen, only fibrous dysplasia of the bone was found. The patient was disease free at his 29 month follow up check up.