Increasing evidence suggests a role for activated T cells and cytokines in the regulation of eosinophilic inflammation in asthma. In this study, we investigated the distribution of leukocytes, lymphocytes, their activation state, and the cytokine profile in BAL from 10 atopic asthmatics with positive skin prick tests and elevated specific IgE levels to birch or grass pollen. Using segmental allergen challenge, 250 PNU of the appropriate allergen or saline were instilled into different segments, which were lavaged 10 min (10 min) and 18 h (18 h) after allergen challenge or 18 h after saline challenge (C). In peripheral blood the number of neutrophils and activated IL-2R+/CD4+ T cells increased significantly 18 h after allergen provocation; there was no change in eosinophils, other leukocytes, or lymphocyte subsets. In contrast, numbers of eosinophils, neutrophils, and IL-2R+/CD4+ T cells increased significantly in BAL samples at 18 h. The numbers of neutrophils and eosinophils were not significantly different in the lavage performed at 10 min and at C. Analysis of cytokines in concentrated BAL fluid revealed significantly increased levels of IL-5, IL-2, IL-1, TNF-alpha, IL-6, IL-8, and GM-CSF, but not of IL-4 and IFN-gamma at 18 h compared with those at C and at 10 min. The correlation between IL-5 levels, eosinophil numbers, and activated T cells supports a role for T-cell-derived IL-5 in causing tissue eosinophilia in allergic asthma.