We report a rare case of primary synovial sarcoma of the kidney. A woman in her 70s, who presented with a history of right abdominal pain, was found, by abdominal CT, to have a tumor arising from the right kidney. Nephrectomy was performed. The histopathological diagnosis was synovial sarcoma. Imprint cytology of the tumor showed numerous small isolated atypical cells. The cells were monotonous and spindle-shaped, with indistinct cell borders. They had round- to oval-shaped nuclei, thin nuclear membranes, extremely fine granular nuclear chromatin, with the nuclei containing one or two small conspicuous nucleoli. The results of immunohistochemical analysis and DNA analysis of the tumor cells were consistent with the diagnosis of synovial sarcoma. Synovial sarcoma of the kidney is extremely rarely diagnosed by cytology.
