Background: IL-10 is an anti-inflammatory cytokine released from various cells, including T cells. The role of IL-10 in asthma pathogenesis remains uncertain. Allergen inhalation by atopic asthmatic subjects results in 2 bronchoconstrictor phenotypes: isolated early response and dual response. Persistence of allergen-induced airway inflammation is a feature of dual responders. Objectives: The kinetics of IL-10 production in circulating T cells were investigated to examine a potential role of IL-10 in allergen-induced responses and airway inflammation. Methods: Fourteen subjects with mild asthma (7 isolated early and 7 dual responders) were challenged with allergen. PBMCs taken before and 24 hours after allergen challenge were processed for intracellular IL-10 staining with fluorescent-conjugated anti-IL-10 antibody. The frequency of IL-10-producing cells was assessed for CD4⁺ and CD8⁺ T-cell subsets by using flow cytometry. Results: Before allergen administration, the frequency of IL-10-producing CD4⁺ cells was significantly higher in dual than in isolated early responders. IL-10-producing CD4⁺ cells significantly increased after allergen in early responders, whereas IL-10-producing CD4⁺ cells significantly decreased in dual responders. Simultaneous assessments of IL-5-producing T cells did not show any differences between each group before or after allergen administration. Conclusions: These results suggest that the contrasting profiles of IL-10 production may be associated with the different time course of allergen-induced airway inflammation between allergen-induced early and dual responders. (J Allergy Clin Immunol 2002;109:281-6.)