Grass pollen immunotherapy is the only treatment for hayfever that is both effective and confers long‐term benefit. Immunotherapy may act by altering the local nasal mucosal T helper type 2 (Th2) to type 1 (Th1) cytokine balance either by down‐regulation and/or immune deviation of T‐lymphocyte responses. There is controversy as to whether these changes are detectable in peripheral blood. We therefore examined both local nasal and peripheral T‐cell responses to allergen exposure in the same subjects before and after immunotherapy. In a double‐blind trial of grass pollen immunotherapy, nasal biopsies were obtained at baseline and during the peak pollen season following 2 years of immunotherapy. Placebo‐treated patients showed a seasonal increase in CD3⁺ T cells (P = 0·02) and in interleukin‐5 (IL‐5) mRNA⁺ cells (P = 0·03) and no change in interferon‐γ (IFN‐γ ) mRNA⁺ cells (P = 0·2) in the nasal mucosa. In contrast, in the immunotherapy‐treated group, there were no changes in the number of CD3⁺ T cells (P = 0·3) and IL‐5 mRNA⁺ cells (P = 0·2) but a significant increase in the number of IFN‐γ mRNA⁺ cells (P = 0·03). Furthermore, clinical improvement in the immunotherapy‐treated group was accompanied by a seasonal increase in the ratio of IFN‐γ to IL‐5 mRNA⁺ cells in the nasal mucosa (P = 0·03). In contrast, there were no significant changes in peripheral T‐cell proliferative responses or cytokine production for IFN‐γ or IL‐5 in response to grass pollen either within or between the two treatment groups. We conclude that successful grass pollen immunotherapy was associated with an increase in the ratio of IFN‐γ to IL‐5 mRNA⁺ cells in the nasal mucosa, whereas these changes were not reflected by alterations in peripheral blood T‐cell proliferative responses or cytokine production before/after treatment.