['Regulatory T (Treg) cells play an important role in controlling allergic inflammation. The transcription factor Foxp3 regulates the development and function of natural and adaptive CD4(+)CD25(+) Treg cells.', 'We sought to examine the effect of grass pollen injection immunotherapy on the numbers of Foxp3(+)CD4(+) and Foxp3(+)CD25(+) T cells in and out of season and their expression of IL-10 in the nasal mucosa of patients with hay fever.', 'Nasal biopsy specimens were obtained from untreated patients with hay fever, participants with grass pollen allergy who had received 2 years of immunotherapy, and healthy control subjects. Dual-immunofluorescence microscopy was used to enumerate and colocalize Foxp3 expression to CD4(+) and CD25(+) T cells in the nasal mucosa. Triple staining was performed to colocalize Foxp3(+) cells to CD3(+)CD25(+) and CD3(+) IL-10-expressing cells.', 'At peak season, numbers of Foxp3(+)CD25(+) (P = .02) and Foxp3(+)CD4(+) (P = .03) cells were significantly increased in the nasal mucosa of immunotherapy-treated patients compared with numbers before treatment. Foxp3(+)CD25(+) (P = .03) and Foxp3(+)CD4(+) (P = .04) cells were also greater in immunotherapy-treated patients out of season compared with those in untreated patients with hay fever. Within the immunotherapy-treated group, 20% of CD3(+)CD25(+) cells expressed Foxp3, and 18% of Foxp3(+)CD3(+) cells were IL-10 positive.'] Does grass pollen immunotherapy induce Foxp3-expressing CD4+ CD25+ cells in the nasal mucosa?