Stem cells of acute myeloid leukemia (AML) have been identified as immunodeficient mouse-repopulating cells with a Lin(-)CD34(+)38(-) phenotype similar to normal hematopoietic stem cells. To identify the leukemia-propagating stem cell fraction of Philadelphia chromosome-positive (Ph(+)) leukemia, we serially transplanted human leukemia cells from patients with chronic myeloid leukemia blast crisis (n = 3) or Ph(+) acute lymphoblastic leukemia (n = 3) into NOD/SCID/IL-2Rgammac(-/-) mice. Engrafted cells were almost identical to the original leukemia cells as to phenotypes, IGH rearrangements, and karyotypes. CD34(+)CD38(-)CD19(+), CD34(+)38(+)CD19(+), and CD34(-)CD38(+)CD19(+) fractions could self-renew and transfer the leukemia, whereas the CD34(-)CD38(+)CD19(+) fraction did not stably propagate in NOD/SCID mice. These findings suggest that leukemia-repopulating cells in transformed Ph(+) leukemia are included in a lineage-committed but multilayered fraction, and that CD34(+) leukemia cells potentially emerge from CD34(-) populations.