A Japanese patient with adult T-cell leukemia-lymphoma (ATL) showed a disease progression from the smoldering type to the chronic type and finally to the acute type. The patient was variously treated, including 2'-deoxycoformycin, with some beneficial effects. During the chronic type he developed a composite lymphoma consisting of T-cell lymphoma (ATL) of medium-sized cells and B-cell lymphoma of diffuse large cell type. At that time, he also suffered from miliary tuberculosis and adenovirus type 11-induced hemorrhagic cystitis, indicating that he was in a marked immunodeficient state. Southern-blot analysis revealed that the two malignancies have distinct clonal origin on the basis of the following results: (1) clonally rearranged T-cell receptor beta-chain gene (TcR-beta gene) and germline configuration of immunoglobulin heavy chain gene (IgH gene) in ATL leukemic cells, (2) clonal rearrangement of IgH gene in lymphoma cells, indicating a monoclonal B-cell lymphoma, (3) monoclonal integration of HTLV-I provirus in ATL leukemic cells, (4) definite presence and monoclonal origin of EBV genome in lymphoma cells. This is the first report of secondary EBV genome carrying monoclonal B-cell lymphoma in an ATL patient. It is suggested that the immunodeficient state in the patient with ATL allows the emergence of EBV-related B-cell lymphoma.