A 19-month-old female with a right temporal bone tumor was diagnosed with diffuse large B-cell lymphoma by tumor biopsy. Leukemic conversion occurred after chemotherapy for mature B-cell lymphoma. Additional chemotherapy was unsuccessful and resulted in death of the patient owing to disease progression. We detected the KMT2A-MLLT3 fused transcript in the bone marrow of the patient with primary and recurrent cancer. RNA sequencing of the bone marrow with recurrent cancer showed similarities with KMT2A-rearranged leukemia, such as overexpression of MEIS1 and MEF2C, which was attributable to the aggressive course.