Background: A suppression of cell mediated immunity and malnutrition are commonly seen in patients with advanced cancer and it has been reported that chronic inflammation plays a key role in induction of these conditions. MDSC: myeloid-derived suppressor cells, found as a new type of immune suppressor cells that is closely related to chronic inflammation, have reported to be present in blood circulation in patients with cancer. MDSC have been reported to suppress immune function through several pathways. Methods: Peripheral blood mononuclear cells (PBMC) were collected from 18 normal healthy volunteers, and 53 patients with gastrointestinal cancer including 5 patients with esophageal, 16 with gastric, 26 with colorectal, 3 with bile duct, 1 with pancreatic and 1 with hepatocellular carcinomas, and these cells were used for the detection of MDSC (CD11b+CD14-CD33+) by flow cytometry. PBMC was also used for the PHA-blastogenesis of lymphocytes which is a marker of cell mediated immunity (stimulation indices: SI). Results: MDSC(%PBMC) of whole patients with gastrointestinal cancer, those with gastric cancer and those with colorectal cancer were higher than those of healthy volunteer (3.76+4.90%, 5.18+1.86%, 4.21+1.43% vs 1.59+1.08%, p<0.05, p<0.05, p<0.10, respectively). MDSC of patients with gastrointestinal cancer were also inversely correlated to the SI (r=-0.271, p<0.05) and to the serum concentrations of total protein (r=-0.490, p<0.005). It also tended to correlate to neutrophil counts (r=0.287, p<0.10) and neutrophil/lymphocyte ratios (NLR, r=0.623, p=0.10), and inversely did to lymphocyte counts (r=-0.251, p<0.10).Thus MDSC was successfully detected in peripheral blood and was significantly higher in patients with gastrointestinal cancer. It was related to the inhibition of cell-mediated immunity, hypoaproteinemia and inflammatory markers such as NLR. Conclusions: MDSC seemed to work in the mechanisms of suppression of immune reaction and malnutrition, typically seen in cancer cachexia, in patients with gastrointestinal cancer and may be important as a marker of advancement of malignant diseases.