We herein present the case of an acute monoblastic leukemia infant who relapsed with acute lymphoblastic leukemia (ALL). Complete remission was achieved after an acute myeloblastic leukemia-directed chemotherapy, but the patient relapsed with pro-B ALL. As he did not respond to acute myeloblastic leukemia-type reinduction therapy, he was switched to ALL-type chemotherapy, and a complete remission was achieved. Unrelated cord blood stem cell transplantation was performed, but he relapsed with pro-B ALL again. After he received ALL-type chemotherapy, he underwent another bone marrow transplant from his human leukocyte antigen mismatch mother, and has been free from recurrence for over 8 months.