The study investigates the role of IL-17 in promoting tumor growth through an IL-6–Stat3 signaling pathway. IL-17, a cytokine produced by Th17 cells, is elevated in various cancers but its contribution to tumor growth is unclear. The researchers found that IL-17 promotes tumor growth in B16 melanoma and MB49 bladder carcinoma models, which is reduced in IL-17−/− mice but accelerated in IFN-γ−/− mice. Adoptive transfer studies and analysis of the tumor microenvironment suggest that CD4+ T cells are the primary source of IL-17. IL-17 enhances tumor growth by directly affecting tumor cells and tumor-associated stromal cells, which bear IL-17 receptors. IL-17 induces IL-6 production, which activates oncogenic Stat3, up-regulating prosurvival and proangiogenic genes. The Th17 response thus promotes tumor growth via the IL-6–Stat3 pathway. The findings highlight the complex role of IL-17 in cancer and suggest that targeting this pathway could be a potential therapeutic strategy.The study investigates the role of IL-17 in promoting tumor growth through an IL-6–Stat3 signaling pathway. IL-17, a cytokine produced by Th17 cells, is elevated in various cancers but its contribution to tumor growth is unclear. The researchers found that IL-17 promotes tumor growth in B16 melanoma and MB49 bladder carcinoma models, which is reduced in IL-17−/− mice but accelerated in IFN-γ−/− mice. Adoptive transfer studies and analysis of the tumor microenvironment suggest that CD4+ T cells are the primary source of IL-17. IL-17 enhances tumor growth by directly affecting tumor cells and tumor-associated stromal cells, which bear IL-17 receptors. IL-17 induces IL-6 production, which activates oncogenic Stat3, up-regulating prosurvival and proangiogenic genes. The Th17 response thus promotes tumor growth via the IL-6–Stat3 pathway. The findings highlight the complex role of IL-17 in cancer and suggest that targeting this pathway could be a potential therapeutic strategy.