It has been reported that ectopically expressed interleukin-17 (IL-17) in tumor cells suppresses tumor progression through enhanced antitumor immunity in immune competent mice or promote tumor progression through an increase in inflammatory angiogenesis in immune-deficient mice. The role of endogenous IL-17 in tumor immunity remains undefined. Here we showed that tumor growth and lung metastasis were enhanced in IL-17–deficient mice, associated with decreased interferon-γ⁺ natural killer cells and tumor specific interferon-γ⁺ T cells in the tumor draining lymph nodes and tumors. Together with the published data showing that in vitro transforming growth factor-β and IL-6–polarized Th17 cells induce tumor regression, our work supports the notion that endogenous IL-17 or/and Th17 cells may play a protective role in tumor immunity.