Grass pollen immunotherapy for the treatment of seasonal allergic rhinitis (‘summer hayfever’) results in improvement in symptoms, a reduction in the early and late phase responses to allergen provocation and decreased tissue eosinophilia. Immunotherapy may act by altering the pattern of cytokine production by allergen-specific T cells from a ‘Th2-type’ (IL-4 and IL-5) profile to a ‘Th1-type’ (interferon-gamma (IFN-γ)) profile. We set out to determine whether clinical improvement following specific allergen immunotherapy is accompanied by reduced production of the pro-eosinophilic and archetypal ‘Th2-type’ cytokine, IL-5. Peripheral blood mononuclear cells (PBMC) were isolated from (i) 13 patients who had received 6 or 7 years' continuous conventional immunotherapy with timothy grass pollen (Phleum pratense); (ii) 14 patients who had received 3 or 4 years of conventional immunotherapy followed by 3 years of placebo treatment; (iii) 12 matched seasonal rhinitic patients who had never received immunotherapy; and (iv) 17 non-atopic normal controls. PBMC were stimulated with 20 μg/ml and 200 μg/ml P. pratense extract, or 10 μg/ml of Mycobacterium tuberculosis purified protein derivative (PPD), at 2 × 10⁶ cells/ml and 5 × 10⁶ cells/ml. IL-5 concentrations in culture supernatants collected after 6 days' culture were measured by ELISA. IL-5 production in response to stimulation with P. pratense extract was highly reproducible and was elevated in both of the immunotherapy treated groups and the untreated rhinitics relative to non-atopic controls (P < 0.005 for each group relative to non-atopic controls, under each of the four conditions tested). However, no significant reduction was observed in IL-5 production when immunotherapy treated patients were compared with untreated rhinitic controls. Moreover, abrogation of the cutaneous late-phase responses to allergen following treatment was not associated with reduced IL-5 production by allergen-stimulated peripheral blood T cells. Reduced IL-5 production by peripheral blood T cells may not be necessary for immunotherapy to be effective. Local immunodulation of T cell responses may play a role in this form of treatment.