Abstract: Objective:To investigate the correlation of the infiltration of myeloid-derived suppressor cells (MDSC) with interleukin- 6 (IL-6) expression and prognosis in patients with breast cancer. Methods:Tumor tissue slices were obtained from patients with breast can-cer who underwent surgery. The expression of IL-6 and the number of MDSCs were detected through immunohistochemistry analysis. The correlation of MDSC infiltration with IL- 6 expression and prognosis in patients with breast cancer was also analyzed. Results:MD-SC infiltration was correlated with the infiltration of lymph nodes, tumor volume, tumor stage, histology grade, and overall survival of the patients. MDSC infiltration was also significantly correlated with IL- 6 expression. Conclusion: IL- 6 secreted by breast cancer cells may induce local infiltration and aggregation of MDSCs. Increased number of MDSCs could be the negative prognostic factor of pa-tients with breast cancer.