A 68-year-old man was admitted to our hospital because of left back pain and systemic lymphadenopathy with hypercalcemia. Serum anti-HTLV-1 antibody was positive. Left cervical lymph node (LN) biopsy revealed proliferation of medium-sized to large CD4-positive atypical cells with modest infiltration of CD20 and Epstein-Barr virus (EBV)-encoded RNA dual-positive atypical large cells. Monoclonal integration of HTLV-1 proviral DNA, plus clonal rearrangement of the T-cell receptor chain gene and the immunoglobulin heavy chain gene, were detected in the same LN specimen. Composite lymphoma consisting of adult T-cell leukemia/lymphoma (ATL) and EBV positive diffuse large B-cell lymphoma (DLBCL) was diagnosed. He was successfully treated with aggressive chemotherapy including rituximab and attained remission. However, eight months later, he developed right shoulder pain due to multiple bone invasions with bilateral cervical lymphadenopathy. Biopsies of a bone lesion and cervical LN revealed recurrence of ATL alone. The patient died despite salvage chemoradiotherapy. These findings suggest that ATL-related immunodeficiency might induce EBV-associated DLBCL.