CD25 is expressed on leukemic cells in 10-20% cases of acute myeloid leukemia (AML), and its expression is associated with poor prognosis. We reevaluated the relationship between CD25 expression and the leukemia-initiating cell (LIC) properties of AML using a patient-derived xenograft model. We divided lineage marker-negative (Lin-) CD34+CD38- or Lin-CD34+ cells from CD25-positive AML into CD25-positive and -negative populations, and then transplanted each population into NOD.Cg-PrkdcscidIl2rgtm1Wjl/Sz mice. Leukemic engraftment was observed with both CD25-positive and -negative populations from three of nine CD25-positive AML patients. In two of those three patients, CD25-positive and -negative Lin-CD34+ cells engrafted at the primary transplantation led to leukemic engraftment at the secondary transplantation, in which engrafted cells contained both CD25-positive and -negative Lin-CD34+ AML cells. In an in vitro culture system, expression of CD25 was considerably induced in the CD25-negative population of Lin-CD34+ cells from two cases of CD25-positive AML. In one case, CD25-positive Lin-CD34+ cells gave rise to CD25-negative as well as -positive CD34+ cells. These observations suggest that there exist CD25-positive and -negative populations that can reconstitute CD25-positive AML in a patient-derived xenograft model, and that CD25 expression fluctuates in the LICs of AML.