Adult T-cell leukemia/lymphoma (ATL) is the commonest lymphoid malignancy in adult Jamaicans, reflecting the role of the causative agent, human T-cell lymphotrophic virus type I (HTLV-I), in altering the pattern of non-Hodgkin lymphoma in an endemic area. A total of 126 cases of ATL were registered in Jamaica between January 1985 and July 1995. There were 65 male and 61 female patients (male-female ratio, 1:1), with a mean age of 43 years (17-85 years). The majority of cases were acute subtype (46.8%), followed by lymphoma (27%), chronic (20.6%), and smoldering (5.6%) types. The disease is associated with a high mortality, with only five of the 126 patients currently alive. The median survival rate is 20 weeks. The epidemiologic, clinical, and laboratory features are similar to those reported in Japan and Brazil, except that the mean age of patients, identical in Jamaica and Brazil (43 years), is 11 years younger than that in Japan (54 years). Given the theory that ATL develops after a long incubation period after early life exposure to HTLV-I facilitated by mother-to-child transmission via breast milk, and the fact that mothers of ATL patients have a high incidence of HTLV-I seropositivity, it would appear that reduction in the incidence of this disease could be achieved by methods aimed at preventing this mode of transmission.