IL-23 and Th17 cells producing IL-17A and IL-22 are found in excess in skin affected by psoriasis. Previous studies showed that IL-22, but not IL-17A, mediates psoriasis-like epidermal hyperplasia following recombinant murine (rm) IL-23 injections into skin. To further investigate the role of IL-17A, ears of mice were injected with rmIL-23. Investigators blinded to treatment conditions and mouse genotypes measured ear swelling, epidermal thickness, and cytokine expression. In wild-type (WT) mice, rmIL-23 induced ear swelling (p< 0.001, all p values versus saline), epidermal hyperplasia by histology (p< 0.001) and confocal microscopy (p< 0.004), and expression of both IL-17A and IL-22. As expected, rmIL-23 injections into IL-22−/− mice resulted in relatively little ear swelling (p< 0.09) and epidermal hyperplasia (p< 0.51 by histology and p< 0.75 by confocal microscopy). Notably, rmIL-23 injections into IL-17A−/− mice produced little ear swelling (p< 0.001, versus IL-23–injected WT mice) and epidermal hyperplasia (p< 0.001 by histology and p< 0.005 by confocal microscopy), even though IL-22 was readily induced in these mice. Furthermore, systemic delivery of blocking Abs directed against either IL-22 or IL-17A completely inhibited IL-23–induced epidermal hyperplasia in WT mice. These results demonstrate that IL-17A, like IL-22, is a downstream mediator for IL-23–induced changes in murine skin and that both of these Th17 cytokines are necessary to produce IL-23–mediated skin pathology. IL-17A may represent an attractive therapeutic target in individuals with psoriasis by blocking downstream effects of IL-23.