Background In subjects with psoriasis, inflammation and epidermal hyperplasia are thought to be controlled by T cell–derived cytokines. Evidence suggests that the TH 17 cell cytokine IL-17A (IL-17) might play a role in disease pathogenesis. Objective We sought to understand the effect that neutralization of IL-17 has on the clinical features of psoriasis and to understand the role that IL-17 has in inflammatory pathways underlying psoriasis in human subjects. Methods We examined skin lesions obtained from 40 subjects participating in a phase I, randomized, double-blind, placebo-controlled trial of the anti–IL-17 mAb ixekizumab (previously LY2439821) in which subjects received 5, 15, 50, or 150 mg of subcutaneous ixekizumab or placebo at weeks 0, 2, and 4. Results There were significant dose-dependent reductions from baseline in keratinocyte proliferation, hyperplasia, epidermal thickness, infiltration into the dermis and epidermis by T cells and dendritic cells, and keratinocyte expression of innate defense peptides at 2 weeks. By week 6, the skin appeared normal. Quantitative RT-PCR and microarrays revealed an ablation of the disease-defining mRNA expression profile by 2 weeks after the first dose of study drug. The effect of IL-17 blockade on expression of genes synergistically regulated by IL-17 and TNF-α was of higher magnitude at 2 weeks than in prior studies with TNF-α antagonism. Conclusion Our data suggest that IL-17 is a key “driver” cytokine that activates pathogenic inflammation in subjects with psoriasis. Neutralizing IL-17 with ixekizumab might be a successful therapeutic strategy in psoriasis.