We demonstrate two conventional chemotherapy-resistant cases of acute promyelocytic leukemia (APL) who were successfully treated with macrophage-colony stimulating factor (M-CSF). Case no 1 was a 40-year-old woman who was made diagnosis of APL on June, 1992, and treated repeatedly with a conventional chemotherapy, BHAC-DMP regimen, resulting in complete remission on October, 1992. After a couple of years, she had relapse with marked growth of APL cells in bone marrow. She was treated with BHAC-AMP and modified B-triple V but could not obtain remission. Case no 2 was a 36-year-old-man with APL who was treated with BHAC-DMP and BHAC-AMP and modified B-triple V therapy. These three conventional chemotherapy regimen were not effective for him. Eight million units of human native M-CSF was administered intravenously for 14 days after the last BHAC-AMP therapy in case no 1, and for 5 days after the last modified B-triple V therapy in case no 2. After the therapy, APL cells in peripheral blood or bone marrow of both patients disappeared completely and normal hemopoietic cells increased, obtaining in complete remission in both cases. These successful cases treated with M-CSF combining chemotherapy may suggest a new therapeutic strategy for APL in addition to all-trans retinoic acid.