Background: We have previously shown increased expression of the CD4⁺ cell chemoattractant IL-16 in bronchial mucosa of patients with asthma. We investigated the effects of allergen challenge on airway IL-16 expression. Methods: We investigated the expression of IL-16 immunoreactivity in bronchial biopsy samples obtained from atopic asthmatic subjects (n = 19) and normal subjects (n = 6) 24 hours after segmental allergen challenge. Control biopsy samples were obtained either at baseline or after diluent challenge. IL-16 expression was correlated to numbers of CD4⁺ cells, CD25⁺ cells, and activated eosinophils. IL-16 bioactivity was assessed in bronchoalveolar fluid obtained from patients with asthma. Results: IL-16 expression was higher in control biopsy specimens obtained from subjects with asthma compared with normal subjects (P < .05). In patients with asthma, numbers of IL-16 immunoreactive cells were significantly higher in biopsy specimens obtained after allergen challenge compared with control biopsy specimens (P < .001). Allergen provocation was associated with release of IL-16 in bronchoalveolar fluid in patients with asthma. In normal subjects, there was no difference in the number of IL-16–immunoreactive cells in biopsy specimens obtained after allergen challenge compared with biopsy specimens obtained after diluent challenge. Allergen challenge was associated with an increase in the numbers of EG2⁺ eosinophils in patients with asthma but not in normal subjects. IL-16 expression correlated with the numbers of CD4⁺ cells and CD25⁺ cells after allergen challenge in asthmatic subjects with a provocative concentration required to decrease the FEV₁ by 20% of its baseline value (PC₂₀FEV₁) < 4 mg/mL. IL-16–immunoreactive cells were identified mainly as T cells and eosinophils in asthmatic subjects after allergen challenge. Conclusion: Endobronchial allergen provocation in atopic asthmatic patients resulted in increased airway expression of IL-16 and release of bioactive IL-16 in airways. IL-16 may contribute to the immunoregulation of the inflammatory infiltrate in the airways in response to antigen. (J Allergy Clin Immunol 2000;106:293-301.)