Non-Hodgkin lymphoma (NHL) is a very rare complication of acute lymphoblastic leukemia (ALL). A Japanese boy presented with B-lineage ALL at the age of 2.5. He was treated with chemotherapy for standard-risk ALL. While he was receiving maintenance treatment 2 years and 9 months after the diagnosis of ALL, diffuse large B cell lymphoma (DLBL) was diagnosed from a biopsy of an abdominal mass. DLBL was treated by surgical resection followed by chemotherapy for 6 months. The patient has been free from the recurrence of ALL or DLBL for 16 months after the development of DLBL.