Background: Although a causal relationship for inflammation and immunity of cancer is more widely accepted today, the precise cell mechanisms mediating this relationship have not been elucidated. Accumulating evidence suggests that myeloid-derived suppressor cells (MDSC), may contribute to the negative regulation of immune responses during cancer and inflammation. IL-17 is a pro-inflammatory cytokine that is primarily secreted by T helper (Th)17 cells and we have reported that IL-17 correlates with immunosuppressive conditions in patients with cancer. Methods: PBMC (peripheral blood mononuclear cells) were harvested from 106 patients including 43 with gastric and 63 with colorectal cancer. PBMC were stimulated with PHA (phytohemagglutinin) and the production of IL-17 was measured by ELISA. MDSC were detected by flow cytometry (CD11b⁺,CD14^-,CD33⁺). The levels of CRP (C-reacting protein) and NLR (neutrophil to lymphocyte ratio) were used as inflammatory markers. Results: Both of MDSC and IL-17 production were increased in patients with advanced stages, and correlated with each other, inflammatory markers and immune suppression. The patients were divided with average levels of MDSC and IL-17 production and the prognosis were analyzed with Kaplan-Meier method. The overall survival of patients with high MDSC or high IL-17 production were significantly worse than those with low MDSC or low IL-17 production, respectively, in patients with stages III and IV, although the differences were not significant in patients with stages I and II. Conclusions: Thus these inflammatory markers are closely related with systemic inflammation involving IL-17 and with immunosuppression driven by MDSC, and are effective prognostic indicators in patients with stages III and IV gastric and colorectal cancer.