The pleiotropic cytokine interleukin 4 (IL-4) has been shown to regulate many processes thought to be important in the allergic diathesis. To determine the mechanism(s) by which IL-4 mediates allergic airway responses to inhaled allergens, we compared the effects of antigen sensitization and challenge on the development of allergic airway responses in mice in which the gene for the signal transducer and activator of transcription factor 6 (Stat6) was disrupted to those of their wild-type littermates. Strikingly, Stat6-deficient mice failed to develop airway hyperresponsiveness (AHR), which was observed in their wild-type littermates after allergen provocation. Moreover, antigen-induced increases in mucus-containing cells were found to be completely Stat6 dependent. Consistent with the lack of Th2 cytokine responses in Stat6-deficient mice, no ovalbumin-specific immunoglobulin (Ig)E was detected in their serum. In contrast, Stat6 signaling only partially mediated antigen-induced eosinophilia with no role in vascular adhesion molecule 1 expression. These results indicate that Stat6 signal transduction is critical in the development of allergen-induced AHR and that agents that specifically inhibit this pathway may provide a novel strategy for the treatment of allergic disorders.