Tertiary lymphoid organs (TLOs) are structures that resemble lymph nodes that can be found in chronically inflamed tissues in a number of inflammatory conditions. TLOs can host ongoing immune responses that can potentially contribute to disease pathogenesis. The development of TLOs has been thought in large part to recapitulate the development of secondary lymphoid organs, but the regulation of TLO development has not been well understood. In a recent article in Nature Immunology, Troy Randall and colleagues1 identify IL-17, a cytokine that contributes to the pathogenesis of a number of inflammatory diseases, as a novel critical player in TLO formation in inflamed lungs.