Effector Th17 cells are a major source of IL-17, a critical inflammatory cytokine in autoimmune diseases and in host defenses during bacterial infections. Recently, splenic lymphoid tissue inducer-like cells have been reported to be a source of T cell independent IL-17. In this study, we report that the immune system contains a unique set of natural occurring IL-17 producing cell, "natural" Th17 (nTh17), which are a memory-like T cell subset. The nTh17 cells can develop in the absence of the IL-6/STAT3 signaling axis required by inducible Th17 cells. The nTh17 cell population is distinct from conventional inducible Th17 cells, since nTh17 cells express substantial amounts of IL-17A (IL-17), but not IL-17F, under the control of the master regulator, RORgammat. The nTh17 cells simultaneously produce IFN-gamma. DO11.10 transgenic mice with a Rag(-/-) background (DO11.10 Rag(-/-)) lack nTh17 cells, and, following intranasal administration of OVA, IL-17-dependent neutrophil infiltration occurs in DO11.10 transgenic mice, but not in DO11.10 Rag(-/-) mice. The impaired neutrophil-dependent airway response is restored by adaptive transfer of nTh17 cells into DO11.10 Rag(-/-) mice. These results demonstrate that a novel T cell subset, nTh17, facilitates the early phase of Ag-induced airway responses and host defenses against pathogen invasion before the establishment of acquired immunity.