The effects of acute allergen exposure on bronchoalveolar lavage cells and mediators and mucosal inflammatory cells were evaluated in 10 subjects with atopic asthma who underwent lavage and biopsy 24 hours after segmental endobronchial allergen challenge. Increased numbers of bronchoalveolar lavage eosinophils were retrieved from the allergen-challenged sites compared with the saline-challenged sites (mean 21.4 vs 1.5 × 10³ cells/ml; p < 0.02). Numbers of neutrophils and proportions of CD4⁺, CD8⁺, CD25⁺, and HLA-DR⁺ T cells were similar at the saline- and allergen-challenged sites. In contrast to the bronchoalveolar lavage findings, eosinophil numbers were not increased in the bronchial submucosa or epithelium. There was also no significant difference in neutrophils, mast cells, CD3⁺, CD4⁺, or CD8⁺ T cells in the submucosa after allergen challenge, but the number of activated (CD25⁺) T lymphocytes in the mucosa did increase after allergen challenge. Allergen challenge did not induce any significant change in endothelial expression of P-selectin, E-selectin, intercellular adhesion molecule-1, or vascular cell adhesion molecule-1. CD11a⁺ and very late antigen-4⁺ cell numbers were similar in the saline- and allergen-challenged sites. This study suggests that in patients with very mild asthma, local allergen challenge induces persistent bronchoalveolar lavage eosinophilia, but the recruitment process seems to have diminished or ceased by 24 hours. (J ALLERGY CLIN IMMUNOL 1996;98:133-43.)