The cytokine IL-9, derived primarily from T-helper (Th)-9 lymphocytes, promotes expansion of the Th2 subset and is implicated in the mechanisms of allergic asthma. We hypothesize that IL-9 also plays a role in human allergic contact dermatitis (ACD). To investigate this hypothesis, skin biopsy specimens of positive patch test sites from non-atopic patients were assayed using qPCR and immunohistochemistry. Along with Th2 associated cytokines, IFN-γ, IL-4, and IL-17A, expression of IL-9, and PU.1, a Th9-associated transcription factor, were elevated when compared to paired normal skin. Immunohistochemistry on ACD skin biopsies identified PU.1+CD3+, and PU.1+CD4+ cells, consistent with Th9 lymphocytes, in the inflammatory infiltrate. PBMC from nickel-allergic patients, but not non-allergic controls, show significant IL-9 production in response to nickel. Blocking studies with monoclonal antibodies to HLA-DR (but not HLA-A, B, C) or chloroquine significantly reduced this nickel-specific IL-9 production. Additionally, blockade of IL-9 or IL-4 enhanced allergen-specific IFN-γ production. A contact hypersensitivity model using IL-9−/− mice, shows enhanced Th1 lymphocyte immune responses, when compared to WT mice, consistent with our human in vitro data. This study demonstrates that IL-9, through its direct effects on Th1 and ability to promote IL-4 secretion, has a regulatory role for Th1 lymphocytes in ACD.