lnterleukin-4 (IL-4) has been shown to play a crucial role in the pathogenesis of allergic disease including bronchial asthma. In order to investigate the role of IL-4 in airway hyperreactivity, we investigated the effect of inhaled recombinant human IL-4 on airway responsiveness to methacholine and eosinophil numbers in induced sputum in eight patients with allergic asthma using a placebo-controlled study design. Our results demonstrated that in the control experiments receiving vehicle inhalation, methacholine PC20 values did not change nor did the numbers of eosinophils in sputum change from baseline values. In contrast, after IL-4 inhalation, methacholine PC20 fell from baseline (0.43 +/- 1.81 mg/mI) to 0.22 +/- 1.73 mg/mI (p < 0.01) at 24 h, and to 0.21 +/- 1. 74 mg/ml (p < 0.01) at 48 h. Accompanying this increased airway sensitivity was a significant eosinophilia in sputum. Our data indicated that IL-4 increases airway responsiveness by recruiting eosinophils into the airway in patients with allergic bronchial asthma.