Interleukin-2 (IL-2)–inducible T cell kinase (ITK) mediates T cell receptor (TCR) signaling primarily to stimulate the production of cytokines, such as IL-4, IL-5, and IL-13, from T helper 2 (T_H2) cells. Compared to wild-type mice, ITK knockout mice are resistant to asthma and exhibit reduced lung inflammation and decreased amounts of T_H2-type cytokines in the bronchoalveolar lavage fluid. We found that a small-molecule selective inhibitor of ITK blocked TCR-mediated signaling in cultured T_H2 cells, including the tyrosine phosphorylation of phospholipase C–γ1 (PLC-γ1) and the secretion of IL-2 and T_H2-type cytokines. Unexpectedly, inhibition of the kinase activity of ITK during or after antigen rechallenge in an ovalbumin-induced mouse model of asthma failed to reduce airway hyperresponsiveness and inflammation. Rather, in mice, pharmacological inhibition of ITK resulted in T cell hyperplasia and the increased production of T_H2-type cytokines. Thus, our studies predict that inhibition of the kinase activity of ITK may not be therapeutic in patients with asthma.