Type-C RNA tumor viruses have been implicated in the etiology of naturally occurring leukemias and lymphomas of animals. Human T-cell leukemia/lymphoma virus (HTLV) is the first human virus of this class consistently identified in association with a specific type of human leukemia/lymphoma. The isolation of HTLV was made possible by the ability to grow mature T-cells in tissue culture usually with T-cell growth factor (TCGF). We now report a cluster of adult T-cell leukemia/lymphoma among Blacks from the Caribbean in which all eight cases are positive for HTLV virus and/or antibody. These patients have disease that appears indistinguishable from Japanese adult T-cell leukemia/lymphoma which, as we have also reported, is associated with HTLV in over 90% of cases. The finding of HTLV antibodies in some of the normal population in the Caribbean and Japan, and the clustering of a specific form of T-cell leukemia/lymphoma in these virus-endemic areas, suggest that HTLV infection may be associated with the occurrence of a distinctive clinico-pathologic entity.