While allergen specific immunotherapy (SIT) is highly effective for allergic diseases in children, the underlying immunological mechanisms are unclear. Regulatory T (Treg) cells may be crucial in induction of tolerance. Our aim was to investigate the role of CD4+CD25+Foxp3+ T cells and IL-10-secreting type I T regulatory (Tr1) cells in the response to one year of cluster SIT to Dermatophagoides pteronyssinus for allergic rhinitis in children. CD4+CD25+Foxp3+regulatory T cells and IL-10-secreting type I T regulatory (Tr1) cells were analyzed in children allergic to Dermatophagoides pteronyssinus during one year cluster specific immunotherapy (SIT) in a prospective and randomized study. Peripheral blood mononuclear cells (PBMCs) were collected from 25 children receiving SIT and 21 receiving pharmacotherapy. The frequencies of CD4+CD25+Foxp3+ T cells and allergen-specific IL-10+IL-4-, IFN-γ+IL-4-, IL-4+IFN-γ-CD4+ T cells were measured by flow cytometry. Production of IL-4, IFN-r, and IL-10 in supernatants from allergen-stimulated PBMC culture was measured by ELISA. Finally, the suppressive effect of CD4+CD25highTreg cells from both groups was estimated.