Bronchial antigen challenge of sensitized atopic patients with asthma results in an early fall in FEV1, followed in a proportion of patients by a late (4 to 24 hours) fall. The late response is accompanied by an increase in bronchial reactivity, which is widely believed to reflect local influx and degranulation of inflammatory cells, particularly eosinophils, in association with elevated local secretion of cytokines. We hypothesized that the development of a late-phase bronchoconstrictor response and airway eosinophilia after allergen challenge of sensitized atopic patients with asthma is associated with elevated induced sputum concentrations of the eosinophil-active cytokines IL-5 and granulocyte-macrophage colony-stimulating factor and the proinflammatory cytokine tumor necrosis factor-alpha. We counted inflammatory leukocytes and measured cytokine concentrations in induced sputum at baseline and 24 hours after inhalational allergen challenge of 15 atopic patients with asthma who had previously demonstrated a late response. We observed significant increases in the numbers of eosinophils and the concentrations of their granule products, eosinophil cationic protein and eosinophil peroxidase. In contrast, the numbers of neutrophils and concentrations of two of their products, myeloperoxidase and human neutrophil lipocalin, did not significantly change. The numbers of sputum eosinophils correlated with the maximal late-phase fall in FEV1. Concentrations of IL-5 and tumor necrosis factor-alpha, but not granulocyte-macrophage colony-stimulating factor, were significantly elevated after allergen challenge. We conclude that the relatively noninvasive technique of induced sputum production can be used to monitor the effect of bronchial provocation on cytokine concentrations in asthma.