To verify molecular mechanisms by which leukemia stem cells (LSCs) maintain a dormant state, we explored the activity of the major prosurvival signal pathways in CD34+/CD38− compartment, supposed to contain LSCs, and CD34+/CD38+ counterparts from patients with acute myelogenous leukemia (AML, n = 11) by fluorescence-activated cell sorting (FACS). CD34+/CD38− cells expressed a greater amount of p-janus kinase 2 (JAK2) and p-signal transducer and activator of transcription 5 (STAT5) than CD34+/CD38+ counterparts in all patients except for one case. In addition, we found that CD34+/CD38− cells were relatively resistant to cytarabine- and the inhibitor of the fms-like tyrosine kinase 3 (FLT3)-mediated growth inhibition, as measured by the clonogenic assay. Interestingly, blockade of JAK2/STAT5 signaling by the specific JAK2 inhibitor AZ960 stimulated cell cycling in CD34+/CD38− cells in conjunction with downregulation of cyclin-dependent kinase inhibitor p21waf1 and sensitized these cells to the growth inhibition mediated by cytarabine and the FLT3 kinase inhibitor. Moreover, exposure of CD34+/CD38− cells to AZ960 potently induced apoptosis in parallel with downregulation of antiapoptotic protein Bcl-xL, as measured by Western blot analysis. Taken together, JAK2/STAT5 signaling may be a promising molecular target to eradicate CD34+/CD38− leukemia cells in individuals with AML.