A 60-year-old woman was admitted to our hospital with symmetrical arthritis of the knees. During the 2 years preceding admission, she had experienced recurrent arthritis. A histological examination of her synovial tissue showed prominent villous proliferation of the synovial cells, prominent vascularity throughout and an inflammatory infiltrate composed of abnormal mononuclear cells. Three months later, she developed fever, skin eruptions, lymphadenopathy and hepatosplenomegaly. She also had hypercalcemia and there was abnormal lymphocytosis in her blood smears. She was diagnosed as having adult T cell leukemia. Parenteral chemotherapy treatment with adriamycin and cyclophosphamide gave remission of all the manifestations of disease, including arthritis. Her leukemia recurred, however, and she died 6 months after the diagnosis was made.