A 63-year-old woman was admitted to our hospital with high-grade fever, liver dysfunction, and pancytopenia. Computed tomography of the whole body revealed hepatosplenomegaly but no lymphoadenopaties. Bone marrow aspiration showed infiltrations of CD20-positive large atypical B-lymphocytes with severe hemophagocytosis. Although she was a human T-cell leukemia virus type 1 carrier, the atypical lymphocyte in bone marrow had IgH rearrangement but not TCR rearrangement. From these clinical and laboratory data, the patient was diagnosed as having B-cell lymphoma-associated hemophagocytic syndrome (B-LAHS) and treated with R-CHOP chemotherapy (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone). After 4 cycles of R-CHOP, she had achieved complete remission. However, increased numbers of CD4+CD25+ flower cells were observed in peripheral blood and HTLV-1 provirus DNA was detected after 5 cycle of R-CHOP. The patient was diagnosed as adult T-cell leukemia-lymphoma (ATL) complicated by B-LAHS. Our observations suggest that continuous immunosuppressive statement for B-cell lymphoma or the chemotherapy against B-LAHS may induce the development of ATL in an HTLV-1 carrier.