The human T-cell lymphotropic retroviruses share many properties, including a preferred tropism for certain lymphocytes and similar modes of transmission. They also differ significantly from each other and on this basis are divided into two distinct groups: the human T-cell lymphocytropic viruses (HTLVs) and the human immunodeficiency viruses (HIVs). HTLV-I, a transforming virus, is the prototype of the first group (HTLVs). It is associated with adult T-cell leukemia (ATL) and is thought to be involved in a central nervous system disorder referred to as tropical spastic paraparesis. This group also includes HTLV-II, which is associated with a T-cell malignancy different from ATL. A newly isolated human retrovirus, termed HTLV-V, may be the etiologic agent of cutaneous T-cell lymphoma/leukemia, and may also be added to this group. HIV, previously known as HTLV-III or lymphadenopathy virus (LAV), a nontransforming virus, causes acquired immunodeficiency syndrome (AIDS) and is the prototype of the second group (HIVs). New members of this group of nontransforming human retroviruses were isolated from individuals in different countries of Africa. LAV-2, also referred to as HIV-2, was isolated from two AIDS patients in Africa, and the first case of AIDS in the United States caused by HIV-2 was reported in January, 1988. A third member of this group, HIV-3, was detected in Africa.