The balance between pro and antiinflammatory cytokines secreted by T cells regulates both the initiation and perpetuation of inflammatory bowel diseases (IBD). In particular, the balance between interferon (IFN)-gamma/interleukin (IL)-4 and transforming growth factor (TGF)-beta activity controls chronic intestinal inflammation. However, the molecular pathways that evoke these responses are not well understood. Here, we describe a critical role for the transcription factor T-bet in controlling the mucosal cytokine balance and clinical disease. We studied the expression and function of T-bet in patients with IBD and in mucosal T cells in various T helper (Th)1- and Th2-mediated animal models of chronic intestinal inflammation by taking advantage of mice that lack T-bet and retroviral transduction techniques, respectively. Whereas retroviral transduction of T-bet in CD62L(+) CD4(+) T cells exacerbated colitis in reconstituted SCID mice, T-bet-deficient T cells failed to induce colitis in adoptive transfer experiments suggesting that overexpression of T-bet is essential and sufficient to promote Th1-mediated colitis in vivo. Furthermore, T-bet-deficient CD62L(-) CD4(+) T cells showed enhanced protective functions in Th1-mediated colitis and exhibited increased TGF-beta signaling suggesting that a T-bet driven pathway of T cell activation controls the intestinal balance between IFN-gamma/IL-4 and TGF-beta responses and the development of chronic intestinal inflammation in T cell-mediated colitis. Furthermore, TGF-beta was found to suppress T-bet expression suggesting a reciprocal relationship between TGF-beta and T-bet in mucosal T cells. In summary, our data suggest a key regulatory role of T-bet in the pathogenesis of T cell-mediated colitis. Specific targeting of this pathway may be a promising novel approach for the treatment of patients with Crohn's disease and other autoimmune diseases mediated by Th1 T lymphocytes.