A rare population of leukemia cells have the properties of leukemia stem cells (LSCs) and cause resistance to therapy, but their development is not clearly understood. In the current study, we show that a higher resistance to cytotoxic drug (Ara-C) can be developed in the subpopulation of promyelocytic leukemia cells that survived radiation treatment. These drug-tolerant leukemia cells (DTLs) are not observed immediately after radiation despite extensive genetic instability in the cells, but appear in 3 weeks of recovery culture. Moreover, when the single cell-derived clones were examined by clonal trafficking, no correlation between radio-resistant and chemo-resistant leukemic clones was detected, indicating that the resistance is developed by active acquisition of the resistance without clonal predisposition. Interestingly, the DTLs mimicked the characteristics of LSCs exhibiting leukemia-initiating activities and lower levels of reactive oxygen species or a higher level expression of bmi-1 as well as higher resistance to retinoic acid-induced differentiation compared to parental leukemic cells. These studies show that an active reacquisition of stem cell-like properties can occur in the leukemia cells to develop resistance to treatments and that such reacquisition process of leukemic cells occurs in a stochastic manner triggered by radiation stress on leukemic cells.