To investigate the immunopathogenesis of inflammation-associated fibrosis, we analyzed the chronic colitis and late-developing fibrosis occurring in BALB/c mice administered weekly doses of intrarectal 2, 4, 6-trinitrobenzene sulfonic acid. We showed first in this model that an initial Th1 response involving IL-12p70 and IFN-γ subsides after 3 wk to be supplanted by an IL-23/IL-25 response beginning after 4–5 wk. This evolution is followed by gradually increasing production of IL-17 and cytokines ordinarily seen in a Th2 response, particularly IL-13, which reaches a plateau at 8–9 wk. In vitro stimulation studies suggest that this IL-13 production is dependent on IL-23 and IL-25, but not on IL-12p70. We then show that IL-13 production results in the induction of an IL-13R formerly thought to function only as a decoy receptor, IL-13Rα 2, and this receptor is critical to the production of TGF-β 1 and the onset of fibrosis. Thus, if IL-13 signaling through this receptor is blocked by administration of soluble IL-13Rα 2-Fc, or by administration of IL-13Rα 2-specific small interfering RNA, TGF-β 1 is not produced and fibrosis does not occur. These studies show that in chronic 2, 4, 6-trinitrobenzene sulfonic acid colitis, fibrosis is dependent on the development of an IL-13 response that acts through a novel cell surface-expressed IL-13R to induce TGF-β 1. A similar mechanism may obtain in certain forms of human inflammatory bowel disease.