After decades of work, a retrovirus of true human origin has been isolated first from a U.S. adult case of T-cell lymphoma and then from cases from various regions of the world. This virus, named HTLV-I, is strongly associated with a malignant leukemia-lymphoma of mature T-cells. This disease was first clinically characterized in Japan but subsequently found to cluster in the Caribbean region, areas of the U.S. and in other countries. This retrovirus-associated malignancy has an adult onset and usually a rapidly fatal course. Lymphadenopathy, hepatosplenomegaly, cutaneous infiltration and hypercalcemia are found. HTLV-I has been characterized in detail and shown to be an exogenous retrovirus. HTLV-I infection is detected in normal populations and is endemic in restricted areas of Japan, the Caribbean, South America, and the U.S. Cases of ATL tend to cluster in these areas. Future studies will focus on learning (1) how HTLV-I is transmitted from individual to individual; (2) the nature of the mechanism by which HTLV-I transforms T-cells; (3) whether we can use probes from HTLV-I to detect other (related) retroviruses in other human neoplasms.