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Early age at onset of atopy is associated with more severe asthma and increased airway responsiveness (AR); the underlying mechanism is unclear but may involve T cell responses.To test the hypothesis that enhanced T cell responses may be associated with early-onset atopy.In a longitudinal study, atopy was determined in infancy and at 6 and 11 years of age. Individuals were categorized as persistent infant-onset atopy (PIOA), early childhood-onset atopy (ECOA) and later childhood-onset atopy (LCOA). At 11 years of age, peripheral blood T cell cytokine responses, AR, exhaled nitric oxide (FENO) and forced expiratory volume in 1 s were determined.The age at onset of atopy was determined for 60 children, of whom 15 had PIOA, 24 had ECOA and 21 had LCOA. An additional 76 children who were never atopic were also included. T cell responses to house dust mite, including interleukin-5, -9, -10 and tumour necrosis factor α, were higher among children with PIA and ECOA, and lower in children with LCOA, P<0.05. In contrast, those children with LCOA or who were not atopic had the highest IL-10 response to PHA (P=0.014). Children with PIOA and ECOA, but not LCOA, had higher AR and FENO compared with non-atopic children (P<0.05). The group with PIOA were more likely among the atopic children to be admitted to hospital for asthma (P<0.05) and also had lower %FEV1 compared with non-atopic children (P=0.023).Early age at sensitization is associated with enhanced T cell cytokine responses and indices of adverse asthma outcome. T cell cytokine responses might be programmed at the time of initial atopic sensitization.