Background. Sublingual allergen immunotherapy (SLIT) is recognized as both efficacious and safe, especially when using monomeric allergoids. Its mechanism of action is based on the differentiation, activation, and maturation of allergen-specific Regulatory T-cells (Tregs), fundamental for the activation and maintenance of immune tolerance. Tregs, originally identified as a subpopulation of Foxp3 expressing CD4+CD25high T-cells; however, under inflammatory conditions, they are conveniently identified by the surface antigen CD127 and are subtyped as Resting, Activated and Effector Tregs by surface expression of CD45RA, HLA-DR and CD39 that represent markers of differentiation status, inhibitory and recall potential. Hence, in this study, circulating Tregs were characterized in pediatric patients suffering from allergic rhinitis (AR) treated by SLIT and to verify the possible correlation between some Treg subsets with successful SLIT. Methods. Twenty children suffering from mite AR and/or asthma were enrolled. AR severity was assessed by evaluating ACT and ARIA scores at baseline and after 12-months of mite-SLIT. Treg analysis of PBMC pre- and post-SLIT, was based on cytofluorimetric determination of total Tregs, as CD4+CD25highCD39+CD127low/neg, further characterized for CD45RA, HLA-DR and CD39 expression. Results. After SLIT, Resting Tregs were significantly reduced whilst Activated/Effector Tregs CD45RAneg (memory Tregs) resulted increased; moreover, CD39 and HLA-DR expression on Tregs was significantly increased. Notably, the intensity of HLA-DR expression on Tregs positively correlated with the improvement of the clinical scores. Conclusions. Our findings suggest that effective SLIT is associated with re-patterning of the differentiation status of Tregs, particularly with the generation of allergen-specific memory effector Tregs.