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We investigated the effects of cluster specific immunotherapy (SIT) with Dermatophagoides pteronyssinus (Der p) on CD4+CD25+Foxp3+ Treg cells and IL-10-secreting type I T regulatory (Tr1) cells in Der p-sensitized children with allergic rhinitis (AR). We performed a prospective randomized study involving 46 children (aged 8–13 yr), of whom 25 children received Der p-SIT + pharmacotherapy and 21 received only pharmacotherapy, over a period of 1 yr. Prior to and at end of treatment, CD4+CD25+Foxp3+ Treg cells and allergen-specific IL-10+IL-4−, IFN-γ+IL-4−, and IL-4+IFN-γ-CD4+ T cells were measured by flow cytometry. Similarly, IL-4, IFN-γ, and IL-10 in supernatants from allergen-stimulated peripheral blood mononuclear cell (PBMC) cultures were measured by ELISA, and the suppressive effect of CD4+CD25high T cells on cell proliferation and cytokine release was estimated from both groups. Allergen-specific serum IgE and IgG4 were also assessed at the beginning and end of treatment by RAST and ELISA, respectively. The levels of allergen-specific Tr1 cells, IgG4, and allergen-induced IL-10 synthesis from PBMC cultures were significantly increased after SIT for 1 yr compared with baseline levels (p < 0.001 for all), with significant correlation between increased levels of Tr1 cells and improvements in nasal symptoms (r = 0.48, p < 0.05). In contrast, the levels of CD4+CD25+Foxp3+ T cells, allergen-specific Th1 and Th2 cells, the production of IL-4 and IFN-γ, and the function of CD4+CD25high T cells were not altered in either group at the end of treatment. These data suggest that the up-regulation of Tr1 cells may play an important role in SIT and be a useful marker of successful SIT in AR patients.