Acute promyelocytic leukemia (APL) is characterized by a block in differentiation and accumulation of promyelocytes in the bone marrow and blood. The majority of APL patients harbor the t(15:17) translocation leading to expression of the fusion protein promyelocytic-retinoic acid receptor α. Treatment with retinoic acid leads to degradation of promyelocytic-retinoic acid receptor α protein and disappearance of leukemic cells; however, 30% of APL patients relapse after treatment. One potential mechanism for relapse is the persistence of cancer “stem” cells in hematopoietic organs after treatment. Using a novel sorting strategy we developed to isolate murine myeloid cells at distinct stages of differentiation, we identified a population of committed myeloid cells (CD34⁺, c-kit⁺, FcγRIII/II⁺, Gr1^int) that accumulates in the spleen and bone marrow in a murine model of APL. We observed that these cells are capable of efficiently generating leukemia in recipient mice, demonstrating that this population represents the APL cancer–initiating cell. These cells down-regulate the transcription factor CCAAT/enhancer binding protein α (C/EBPα) possibly through a methylation-dependent mechanism, indicating that C/EBPα deregulation contributes to transformation of APL cancer–initiating cells. Our findings provide further understanding of the biology of APL by demonstrating that a committed transformed progenitor can initiate and propagate the disease.