We have identified a Japanese patient with adult T-cell leukemia (ATL) whose T cells in vitro produced the human T-cell leukemia virus (HTLV). This patient presented with lymphomatous arthritis and leukemia and subsequently developed skin lesions. Skin invasion by malignant T-cells was angiocentric and produced vessel wall destruction, resulting in necrotic cutaneous tumor nodules. Malignant T cells in peripheral blood, skin, and joint prior to culture in vitro did not express p19 HTLV-associated antigen. However, by electron microscopy, intracellular type C viral particles were seen in skin-infiltrating T cells. Peripheral blood malignant cells after 7 days in culture with T-cell growth factor-supplemented media expressed p19 antigen, and type C virus particles were seen by electron microscopy to be budding from malignant T lymphocytes. Mitomycin-C-treated peripheral-blood T cells induced the transformation of cord blood T cells into HTLV-infected p19+ T cells. The demonstration of HTLV in malignant T cells from our patient confirms the association of HTLV with Japanese adult T-cell leukemia. Moreover, HTLV may be associated with a vasculitis-arthritis syndrome.