Thrombocytopenia, anasarca, myelofibrosis, renal dysfunction and organomegaly (TAFRO) syndrome is a variant of Castleman's disease recently identified in Japan. A 73-year-old man was diagnosed with TAFRO syndrome according to clinical findings, and his symptoms improved after corticosteroid therapy. Ten months later, lymphadenopathy worsened during tapering of corticosteroids. Histological findings of abdominal lymph nodes showed diffuse large B-cell lymphoma. After 6 cycles of R-CHOP therapy, he has remained in sustained complete remission. This is a rare case of the development of malignant lymphoma during the treatment of TAFRO syndrome, which suggests an association between diffuse large B-cell lymphoma and TAFRO syndrome.