Human T-cell leukemia/lymphoma virus is a unique family of T-cell tropic, human, type-C retroviruses. The discovery of this class of retroviruses provides the first proven link between retroviruses and cancer in humans. This virus is endemic in certain parts of the world, including the southeastern United States, and is associated with the development of adult T-cell leukemia/lymphoma, a fulminant lymphoproliferative disorder frequently accompanied by opportunistic infections and hypercalcemia. Over the last few years, major advances have been made in understanding the clinical, epidemiologic, molecular biologic, and immunologic features of this unique class of human RNA tumor viruses.