Abstract: Objective: To investigate the change of the mast cells(MCs) and its relationship with the expression of Th1 cytokines(IL-2,IFN-γ),and to elucidate their possible roles in visceral hypersensitivity in patients with irritable bowel syndrome(IBS).Methods: The mucosa specimens were taken from the sigmoid colon of 10 normal controls and 30 patients with post-infective IBS.The MCs,IL-2 and IFN-γ were stained by histochemical and immunohisto-chemical methods respectively,and color image analyzer was applied to investigate the results qualitatively and quantitatively.The ultrastructure of cells in colic mucosa and the changes of the MCs were studied through an ultra-microscopy using situs embedding technique and a light microscopic study in serial sections.The results were analyzed by image analysis software.Results: The number of MCs in the sigmoid colon of patients with IBS was significantly elevated(P<0.05),and MCs in IBS presented prominent variations.Significant increase of the IL-2 and IFN-γ was found in the intestinal mucosa of IBS compared to the control.Conclusions: The Expressions of MCs,IL-2 and IFN-γ are significantly increased in patients with post-infective-IBS,which indicates that the activation of colonic MCs is involved in the visceral sensitization.The MCs may be the middle link of immunity mechanism and nerve mechanism.High expressions of IL-2 and IFN-γ may partly explain the sensitization of the internal organs.Infection is not the only factor leading to the changes of mast cells in patients with IBS.