Background: Recent studies have shown that myeloid-derived suppressor cells (MDSC), which have been identified in most patients, are potent suppressors of T cell activation. MDSC have been reported to be correlated with tumorigenesis and the progression of tumors. We have previously reported significant correlations of MDSC with nutritional damage and inflammation in many types of cancer. In this study the quantitative detection of MDSC in peripheral blood was analyzed in the prognosis in patients with gastric cancer. Methods: We tested PBMCs from 29 preoperative patients with gastric cancer and 18 healthy volunteers using flow cytometric analysis (CD14^-CD11b⁺CD33⁺). Markers for nutritional status (serum levels of total protein), inflammation (NLR: neurphil/lymphocyte ratio, serum levels of vascular endothelial growth factor: VEGF) and Th2-polarization (PBMC’s production of IL-6 and IL-10) were measured in this study. The prognosis of the patients was assessed by Kaplan-Meier tests for correlation with levels of MDSC. Results: MDSC levels in 29 preoperative patients with gastric cancer were significantly higher than in normal volunteers. These levels were not associated with pathological characteristics. However they were significantly correlated with the production of IL-6 and IL-10, VEGF levels and NLR. A significant inverse correlation with levels of total protein was obtained. MDSCs were increased in stage IV patients compared with healthy volunteer and 2-year survival rate of the patients with higher levels of MDSC (> 1.104%PBMC) was significantly poorer (median OS, 498 vs 473 days; p = 0.048), while there was no significant difference in patients with stages I, II, and III. Conclusions: These data suggest that increased MDSC is an effective biomarker for immunosuppression due to Th2-polarization, nutritional impairment, systemic inflammation and poor prognosis in advanced patients with gastric cancer.