© 2018 American Association for Cancer Research. Purpose: High-fat diet (HFD) could induce prostate cancer progression. The aim of this study is to identify mechanisms of HFD-induced prostate cancer progression, focusing on inflammation. Experimental Design: We administered HFD and celecoxib to autochthonous immunocompetent Pb-Creþ;Pten(fl/fl) model mice for prostate cancer. Tumor growth was evaluated by tumor weight and Ki67 stain, and local immune cells were assessed by flow cytometry at 22 weeks of age. Cyto-kines which correlated with tumor growth were identified, and the changes of tumor growth and local immune cells after inhibition of the cytokine signals were evaluated in the mice. IHC analyses using prostatectomy specimens of obese patients were performed. Results: HFD accelerated tumor growth and increased the myeloid-derived suppressor cells (MDSCs) fraction and M2/M1 macrophage ratio in the model mice. Celecoxib-suppressed tumor growth, and decreased both local MDSCs and M2/M1 macrophage ratio in HFD-fed mice. HFD-induced tumor growth was associated with IL6 secreted by prostatic macrophages, as were phosphorylated STAT3 (pSTAT3)-positive tumor cells. Anti-IL6 receptor antibody administration suppressed tumor growth, and decreased local MDSCs and pSTAT3-positive cell fractions in HFD-fed mice. The tumor-infiltrating CD11b-positive cell count was significantly higher in prostatectomy specimens of obese than those of nonobese patients with prostate cancer. Conclusions: HFD increased MDSCs and accelerated prostate cancer tumor growth via IL6/pSTAT3 signaling in the mice. This mechanism could exist in obese patients with prostate cancer. IL6-mediated inflammation could be a therapeutic target for prostate cancer.