Background: Immunotherapy involves the modulation of allergen-specific T-cell responses, either T_H2-to-T_H1 immune deviation or, in bee venom-treated patients, induction of IL-10 production by CD4⁺CD25⁺ T cells. IL-10-producing CD4⁺CD25⁺ regulatory T cells have emerged as potential mediators of immune tolerance in numerous murine models of immunopathology. Objective: The aim of this study was to evaluate the role of IL-10 production and CD4⁺CD25⁺ T cells in the response to grass pollen immunotherapy. Methods: PBMCs were isolated from patients after 1 year of grass pollen immunotherapy and from matched untreated atopic and healthy control subjects. After 6 days of in vitro stimulation with Phleum pratense , production of IL-10, IL-5, IL-4, and IFN-γ and proliferation and numbers of CD4⁺CD25⁺ T cells were measured. T cells were then stimulated for a further 5 hours with phorbol 12-myristate 13-acetate and ionomycin and assessed for intracellular IL-10 by means of flow cytometry. Results: Patients undergoing immunotherapy produced significantly more IL-10 than atopic control subjects (patients undergoing immunotherapy, 116 ± 21 pg/mL [n = 11]; atopic patients, 30 ± 5 pg/mL [n = 11]; P < .001), and the number of CD4⁺CD25⁺ cells identified after allergen stimulation was also greater in the immunotherapy group. The numbers of CD4⁺CD25⁺ T cells correlated positively with activation as measured by proliferation in both of the control groups but not in the immunotherapy group. Moreover, only T cells from patients undergoing immunotherapy were positive for intracellular IL-10, and these were almost exclusively CD4⁺CD25⁺ cells. Conclusion: Grass pollen immunotherapy results in a population of circulating T cells that express the IL-10⁺CD4⁺CD25⁺ phenotype in response to allergen restimulation. (J Allergy Clin Immunol 2003;111:1255-61.)