We tested in B6 mice whether the local expansion of CD4 T cells producing proinflammatory cytokines including IL-17 (Th17 cells) in the colonic lamina propria (cLP) depends on the commensal microflora. High numbers of CD4 Th17 cells were found in the lamina propria of the ileum and colon but not the duodenum, jejunum, mesenteric lymph nodes, spleen, or liver of specific pathogen-free (SPF) mice. The microflora is required for the accumulation of cytokine (IL-17, IFN-γ, TNF-α, IL-10)-producing CD4 T cells in the cLP because only low numbers of cytokine-producing cLP CD4 T cells were found in syngeneic (age-and sex-matched) germfree mice. The fraction of cLP Th17 cells was higher in (type I and type II) IFN-but not IL-4-or IL-12p40-deficient SPF congenics. cLP CD4 Th17 cells produce IL-17 but not IFN-γ, TNF-α, IL-4, or IL-10. cLP CD4 Th17 cells accumulate locally in colitis induced by adoptive transfer of IFN-γ+/+ or IFN-γ−/− CD4 T cells into congenic SPF (but not germfree) RAG−/− hosts. In this colitis model, cLP CD4 T cells that “spontaneously” produce IL-17 progressively increase in number in the inflamed cLP, and increasing serum IL-17 levels appear as the disease progresses. Commensal bacteria-driven, local expansion of cLP CD4 Th17 cells may contribute to the pathogenesis of this inflammatory bowel disease.