Human T-cell leukemia virus (HTLV) is a human type-C RNA tumor virus (retrovirus) previously identified in and isolated from several patients with T-cell leukemias or lymphomas. The known virus isolates from the United States and Japan are closely related and are found in adults with an acute malignancy of mature T cells. A related retrovirus has been found in a patient (Mo) with a somewhat different disease (a T-cell variant of relatively benign hairy cell leukemia). Serum from Mo contains antibodies to the major internal core protein (p24) of HTLV. A T-cell line established from the spleen of Mo expresses HTLV antigens. However, HTLV from Mo is significantly different from all previous HTLV isolates in immunological cross-reactivity tests of p24. The usual prototype HTLV isolate is represented as HTLV-I, and the HTLV from Mo is represented as HTLV-II. Individual members of each subgroup may then be identified by subscript initials of the patient [for example, HTLV-I(CR), HTLV-I(MB), and HTLV-II(Mo)].

Human T (thymus-derived)-cell leukemia/lymphoma virus (HTLV) is the first retrovirus consistently isolated from humans. Seroepidemiologic testing for antibodies to HTLV document the following. (1) HTLV is associated with a spectrum of mature T-cell lymphoreticular neoplasms. (2) HTLV is strongly associated with clusters of adult T-cell leukemia in Japan and a related syndrome, lymphosarcoma T-cell leukemia in the Caribbean. (3) Virus-positive infections from other areas of the world share similar clinicopathologic features, with some overlap with cutaneous T-cell lymphoma (CTCL). Antibodies to HTLV are lacking in most persons with CTCL. (4) Virus-associated malignancy clusters in geographic areas where HTLV infection is prevalent, and virus positivity varies by country, region within country, age, and possibly race and sex. Although preliminary, the epidemiologic data suggest that HTLV is etiologically linked to a specific subtype of mature T-cell malignancy.