A 56-year-old man was admitted complaining of pain in the right tibia. He was diagnosed as having adult T-cell leukemia/lymphoma (ATL/L) in the right knee joint by roentgenographic and histological examination. Monoclonal integration of HTLV-I proviral DNA was demonstrated in the bone tumor cells, although polyclonal integration was observed in the peripheral blood. These results led us to make a diagnosis of ATL with localized growth in the right knee joint. The osteolytic bone change was progressive despite radiation therapy. Complete remission was achieved after amputation of his right lower leg and two courses of chemotherapy. Resection of the localized lesion might be useful in cases of ATL, but further studies are necessary to confirm this conclusion.