Objective To investigate the effect of tumor-associated fibroblasts (TAFs) on non-cancer stcm cclls (NCSCs) dedifferentiating into cancer stem cells (CSCs) in breast cancer.Methods TAFs were purified from 8 cases of primary breast cancer tissues by fluorescence activated cell sorter (FACS) and identified by immunofluorescence.The role of TAFs in the regulation of unsorted breast cancer cells (BCCs) self-renewal ability was evaluated with the sphere formation assay.The non-CD44 +CD24-/low cells sorted by FACS were grown in an adherent culture to evaluate the TAFs induced dedifferentiation of NCSCs.The proportion of CD44 + CD24-/low tumor cells was measured before and after the TAFs conditioned medium (CM) treatment.Results The stromal cells purified were TAFs with identification of vimientin antibodies by immunofluorescence.TAFs effectively enhanced the self-renewal ability of the unsorted BCCs.After co-culture with TAFs,the number of the unsorted BCCs to form spheres was (71.67 ±3.51),and that in the control group was (24.67 ± 1.53,P ＜0.01).The self-renewing ability of the unsorted BCCs was also enhanced by CM.The sphere number of CM and control groups was (74.67 ± 3.06)and (30.33 ±3.21),respectively (P ＜0.01).TAFs also promoted the non-CD44+ CD24-/low subpopulation to dedifferentiate.After processed by CM,the number of the non-CD44 + CD24-/low cells to form spheres was (86.00 ± 3.00),and that was (15.33 ± 1.53) in control group,respectively (P ＜ 0.01).And after CM treatment,the proportion of CD44 + CD24-/low cells was (52.13 ± 0.70) ％,and that in con-trol group was (17.07 ± 0.65) ％ (P ＜ 0.01).Conclusion TAFs effectively enhances the self-renewal ability of breast cancer cells,and promotes NCSCs to dedifferentiate into CSCs in a paracrine way.