Interleukin (IL)-17-producing CD4+ T lymphocytes (T H17 cells) constitute a subset of T-helper cells involved in host defence and several immune disorders. An intriguing feature of TH17 cells is their selective and constitutive presence in the intestinal lamina propria. Here we show that adenosine 5′-triphosphate (ATP) that can be derived from commensal bacteria activates a unique subset of lamina propria cells, CD70highCD11clow cells, leading to the differentiation of TH17 cells. Germ-free mice exhibit much lower concentrations of luminal ATP, accompanied by fewer lamina propria T H17 cells, compared to specific-pathogen-free mice. Systemic or rectal administration of ATP into these germ-free mice results in a marked increase in the number of lamina propria TH17 cells. A CD70 highCD11clow subset of the lamina propria cells expresses TH17-prone molecules, such as IL-6, IL-23p19 and transforming-growth- factor-β-activating integrin-αV and -β8, in response to ATP stimulation, and preferentially induces TH17 differentiation of co-cultured naive CD4+ T cells. The critical role of ATP is further underscored by the observation that administration of ATP exacerbates a T-cell-mediated colitis model with enhanced TH17 differentiation. These observations highlight the importance of commensal bacteria and ATP for TH17 differentiation in health and disease, and offer an explanation of why TH17 cells specifically present in the intestinal lamina propria.