['Ulcerative colitis (UC) is a chronic, relapsing and remitting, inflammatory disorder of the large intestine. Mucosal associated invariant T (MAIT) cells are a member of innate-like lymphocytes found abundantly in the mucosal tissue. The contribution of MAIT cells in the pathogenesis of UC is still unclear; therefore, this study aimed at investigating the role of these cells in patients with UC.', 'The frequency of MAIT cells, as well as the production of cytokines and expression levels of activation markers by these cells in the peripheral blood of UC patients and healthy controls, was analyzed by flow cytometry. MAIT cells were also quantified in colon biopsies of UC patients using a confocal microscope.', 'There was a significant reduction in MAIT cell frequency in the peripheral blood of UC patients compared with healthy controls (P\u2009<\u20090.0001). MAIT cells from UC patients secreted more interleukin (IL)-17 than healthy controls (P\u2009<\u20090.05). The expression levels of CD69 on these cells were correlated with disease activity and endoscopic scores and plasma levels of IL-18. Furthermore, MAIT cells increased in the inflamed mucosa, and their frequency was correlated with clinical and endoscopic disease activity in UC patients.'] Are mAIT cells activated and accumulated in the inflamed mucosa of ulcerative colitis?