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BACKGROUND AND OBJECTIVES The aim of this study was to evaluate the role of T- and B-regulatory cells (Tregs and Bregs) in the pathogenesis of idiopathic granulomatous mastitis (IGM). METHODS This study includes 47 patients with pathologically proven IGM (Group P) and 26 healthy subjects (Group C). The patients in Group P were divided into two groups according to whether their lesions were active (Group PA, n: 21) or in remission (Group PR, n: 26). By using flow-cytometry, the frequencies of CD3+CD4+CD45RA-Foxp3high activated Tregs (aTregs), CD3+CD4+CD45RA-Foxp3low non-suppressive Tregs, CD3+CD4+CD45RA+Foxp3low resting Tregs (rTregs), CD3+CD4+CD25+Foxp3- T-effector cells (Teff), total Tregs and Bregs were analyzed in all subjects. RESULTS The frequency of the Teff cells was statistically higher in Group P when compared with Group C (p =.004). The Foxp3 expression of Treg cells and the frequency of non- suppressive Tregs in Group P were statistically lower than Group C (p =.032 and p =.02, respectively). In addition, Group PR's Foxp3 expressions were statistically lower than Group C (p =.027); Group PR's aTregs ratio was statistically lower than Group PA (p =.021); and the non-suppressive Tregs ratio of Group PR was lower than both Group PA and Group C (p =.006 and p <.0001). No significant differences were seen Bregs and B cell subsets. CONCLUSION Significant changes in Foxp3 expression and Treg subsets were seen in patients with active IGM lesion and in remission. This study shows an intrinsic defect of Tregs in patients with IGM.