Myeloid-derived suppressor cells (MDSCs) are one of the major cell populations responsible for regulating immune responses. MDSCs have been reported to accumulate in the blood, lymph nodes, and at tumor sites in most patients during tumor progression and chronic infection, where they potentially suppress T cell functions. We analyzed MDSCs (CD11b+ CD14- CD33+) in peripheral blood mononuclear cells by flow cytometry in 222 patients with esophageal, gastric, colorectal, hepatocellular, cholangiocellular, pancreatic, breast, ovarian, thyroid, and lung cancer, and 18 healthy volunteers. MDSCs were significantly higher in patients with esophageal, gastric, colorectal, hepatocellular, pancreatic, and breast cancer than in healthy volunteers, and the differences were not significant in patients with cholangiocellular, ovarian, thyroid, and lung cancer. Production of the cytokines IFN-γ and IL-6 in response to phytohemagglutinin was assayed using enzyme-linked immunosorbent assay (ELISA) test kits. Serum concentrations of sIL-2R were measured by ELISA. The percentages of MDSCs in patients with colorectal cancer positively correlated with neutrophil counts and the concentration of sIL-2R(both p<0.05), and inversely correlated with the production of IFN-γ( p<0.0001), serum albumin concentration(p<0.005), and lymphocyte counts (p<0.05). These data suggested that MDSCs are strongly related to chronic inflammation and nutritional impairment.