A 46-year-old man having back pain for 3 weeks showed multiple lymphadenopathy and epidural mass (T7-T10) on physical examination, computed tomography, and magnetic resonance imaging. Initial complete blood cell counts were: Hb, 8.7 g/dL; WBC, 8.79×10⁹/L; and platelets, 74×10⁹/L. Peripheral blood smear showed several small, mature-appearing lymphocytes (62%) (A; Wright stain, ×1,000). Spinal bone biopsy showed diffuse infiltration of large mononuclear cells with prominent nucleoli (B; hematoxylin-eosin stain, ×400), and partial juxtaposition of large cells and small lymphoid cells (C). Large cells were CD10-, CD20-, and CD79a-positive and CD3-, CD5-, and cyclin D1-negative. The patient was diagnosed with diffuse large B-cell lymphoma (DLBL). Further, bone marrow (BM) aspirate showed several small lymphoid cells (37%), and large immature cells with occasional cytoplasmic vacuolations (D; Wright stain, ×1,000). Flow cytometric analysis of BM aspirate demonstrated that small lymphoid cells (CD45+/low side scatter) were CD5-, CD19-, CD20-, and CD23-positive and TdT-, CD10-, and FMC7-negative, consistent with chronic lymphocytic leukemia (CLL). Transformation of CLL to DLBL occurs in 1-10% of CLL cases and has a poor prognosis. The clonality of both neoplasms was not determined in this case. Despite several chemotherapy cycles, the patient died 3 months after diagnosing DLBL.