A 78-year-old man complaining of the left knee pain was admitted to our hospital in January 1989. The patient experienced knee pain, swelling and local heat. Physical examination revealed tenderness, swelling local heat, bollotment, and varus deformity in the knee joint. Radiographs demonstrated joint narrowing, spurring formation, and sclerotic changes of subchondral bone in the knee joint. Antibody to human T cell leukemia virus, measured by indirect immunofluorescence, was positive in the

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... um (1: 4096), in the synovial fluid (1: 2048), and in the spinal fluid (1: 32). The atypical lymphocytes compatible with ATL-like cell were observed in synovial fluid. The surgical treatment was managed with synovectomy and unicompartmental prosthetic total knee replacement. The synovial proliferation was observed. The histology of synovium by hematoxyrin and eosin staining demonstrated villous proliferation, no lining cells, and no lymphoid cell infiltration.