HTLV-I, the first human oncovirus, is a type C retrovirus linked to the development of ATLL. The virus shows a striking ethnogeographic distribution that is only partially understood. Certain populations at high risk for AIDS appear to have a higher incidence of HTLV-I infection. The extended latent period renders present knowledge of the sequelae and natural history of HTLV-I seropositivity incomplete, although recent data suggest that HTLV-I infection may have important implications for blood transfusion, organ transfer, and public health policy. A variety of clinical syndromes have been associated with infection, ranging from an asymptomatic carrier state to acute ATLL with lymphadenopathy, hepatosplenomegaly, hypercalcemia, cutaneous lesions, and systemic immunosuppression. Conventional chemotherapy is marginally effective; innovative approaches to therapy are presently being evaluated.