Human T-cell lymphotropic virus Type I (HTLV-I) is the primary etiologic factor for adult T-cell leukemia/lymphoma (ATL). Although HTLV-I is endemic in japan and the Caribbean islands, the reported clinical and epidemiologic features of ATL in these 2 parts of the world are quite different. ATL has been diagnosed at a younger age and is reported more frequently as the lymphomatous type rather than the acute type with leukemia in the Caribbean basin as compared with the presentation in Japan. In order to characterize ATL in the United States, a registry has been established at the National Cancer Institute for the purpose of recording all cases originally diagnosed in the United States. This registry was utilized to examine the effect of ethnic differences on age of onset and clinical features of ATL, using the same data base. Clinical and laboratory information was obtained from 177 patients suspected of having ATL, who were treated at the National Institutes of Health, or had biological samples sent for evaluation, or were reported in the literature. Histopathologic review and virologic studies were performed by standardized methods. Of 177 patients registered, 127 were considered as having ATL, according to an algorithm combining clinical, pathologic and laboratory features. Presenting features in the confirmed cases consisted primarily of lymphadenopathy (76.6%), hypercalcemia (72.5%), leukemia (82%), skin involvement (48.2%) and hepatomegaly (53.6%). Patients of Japanese ancestry were generally older (median age 63, range 51 to 73 years) than patients of African-American descent (median age 39, range 7 to 75 years) and presented more often with leukemia (90 vs. 69%). Of the 103 cases where country of birth was confirmed, 45 (43.7%) were born in the United States. The prognosis was generally poor (median survival 3.24 months), but rare longterm remissions were documented.