We had shown previously that the prevalence of human T-cell leukemia/lymphoma virus type I (HTLV-I)-antibody positivity is high in Jamaican non-Hodgkin's lymphoma (NHL) patients and that virus-positive patients have the clinical features and poor prognosis of adult T-cell leukemia/lymphoma (ATL). Sixty-two % of 45 NHL patients diagnosed consecutively between 2/1/82 and 1/31/84 and studied prospectively were HTLV-I-antibody positive. Skin involvement (38%), hypercalcemia (44%), and leukemia (40%) were unusually prevalent and there was a strong association (p less than 0.05) with HTLV-I-antibody positivity. Fifty-two % of the patients had bone marrow infiltration, and 74% of these patients were HTLV-I-antibody positive (p = 0.06). Lymphadenopathy (96%), hepatomegaly (60%), and splenomegaly (25%) were detected with about the same frequency as in other series of NHL patients with advanced disease, and 61-88% of these patients were HTLV-I-antibody positive. Patients were classified into those with "typical ATL" (NHL associated with 2 of the 4 features i) hypercalcemia; ii) histologically proven skin infiltration; iii) leukemia; and iv) bone marrow infiltration, providing that the morphology of infiltrating or leukemic cells was characteristic of ATL; those "consistent with ATL" (NHL associated with 1 of these 4 features); and "non-ATL" (NHL without any of these 4 additional features). Thirty-two (71%) of the NHL patients were ATL patients, i.e., had features typical of or consistent with ATL, and 78% of these were HTLV-I-antibody positive. HTLV-I provirus was detected in tumour cells of all HTLV-I-antibody positive patients tested. Three (23%) of the non-ATL patients were HTLV-I-antibody positive. There was no correlation between histopathological features and the clinical classification or HTLV-I-antibody positivity. Median survival of ATL and non-ATL patients was 16 and 53 weeks. Although the disease was usually fulminant, 34% of the ATL patients had a subacute or chronic course. Skin involvement and leukemia were prominent in these patients. Hypercalcemia was the chief prognostic determinant. Median survival of hypercalcemic and normocalcemic ATL patients was 13 and 86 weeks (p less than 0.05). Hypercalcemia caused 10 deaths, infections 12, and death was due to tumour progression in 4 patients. Infections were usually due to pyogenic organisms and only 2 patients had systemic opportunistic infections. Six (27%) of 22 chronic lymphocytic leukemic (CLL) patients were HTLV-I-antibody positive.(ABSTRACT TRUNCATED AT 400 WORDS)