AIM:To investigate the immunological repertoire in the peritoneal cavity of gastric cancer patients.METHODS:The peritoneal cavity is a compartment in which immunological host-tumor interactions can occur.However,the role of lymphocytes in the peritoneal cavity of gastric cancer patients is unclear.We observed 64 patients who underwent gastrectomy for gastric cancer and 11 patients who underwent laparoscopic cholecystectomy for gallstones and acted as controls.Lymphocytes isolated from both peripheral blood and peritoneal lavage were analyzed for surface markers of lymphocytes and their cytokine production by flow cytometry.CD4+CD25high T cells isolated from the patient’s peripheral blood were co-cultivated for 4 d with the intra-peritoneal lymphocytes,and a cytokine assay was performed.RESULTS:At gastrectomy,CCR7 -CD45RA-CD8+ef-fector memory T cells were observed in the peritoneal cavity.The frequency of CD4+CD25 high T cells in both the peripheral blood and peritoneal cavity was elevated in patients at advanced stage[control vs stage Ⅳin the peripheral blood:6.89(3.39-10.4)vs 15.34(11.37-19.31),P<0.05,control vs stageⅣin the peritoneal cavity:8.65(5.28-12.0)vs 19.56(14.81-24.32),P<0.05].On the other hand,the suppression was restored with CD4+CD25high T cells from their own peripheral blood.This study is the first to analyze lymphocyte and cytokine production in the peritoneal cavity in patients with gastric cancer.Immune regulation at advanced stage is reversible at the point of gastrectomy.CONCLUSION:The immunological milieu in the peritoneal cavity of patients with advanced gastric cancer elicited a Th2 response even at gastrectomy,but this response was reversible. AIM: To investigate the immunological repertoire in the peritoneal cavity of gastric cancer patients. METHODS: The peritoneal cavity is a compartment in which immunological host-tumor interactions can occur. However, the role of lymphocytes in the peritoneal cavity of gastric cancer patients is unclear .We observed 64 patients who underwent gastrectomy for gastric cancer and 11 patients who underwent laparoscopic cholecystectomy for gallstones and acted as controls. Lymphocytes isolated from both peripheral blood and peritoneal lavage were analyzed for surface markers of lymphocytes and their cytokine production by flow cytometry. CD4 + CD25high T cells isolated from the patient’s peripheral blood were co-cultivated for 4 days with the intra-peritoneal lymphocytes, and a cytokine assay was performed. RESULTS: At gastrectomy, CCR7-CD45RA-CD8 + ef-fector memory T cells were observed in the peritoneal cavity. The frequency of CD4 + CD25 high T cells in both the peripheral blood and peritoneal cavity was eleva ted in patients at advanced stage [control vs stage IV in the peripheral blood: 6.89 (3.39-10.4) vs 15.34 (11.37-19.31), P <0.05, vs vs stage IV in the peritoneal cavity: 8.65 (5.28-12.0) vs 19.56 (14.81 -24.32), P <0.05] .On the other hand, the suppression was restored with CD4 + CD25high T cells from their own peripheral blood. This study is the first to analyze lymphocyte and cytokine production in the peritoneal cavity in patients with gastric cancer . Immune regulation at advanced stage is reversible at the point of gastrectomy. CONCLUSION: The immunological milieu in the peritoneal cavity of patients with advanced gastric cancer elicited a Th2 response even at gastrectomy, but this response was reversible.