A close relationship between inflammation and colon cancer has been widely accepted, and interleukin (IL)-17A plays an important role in controlling colonic inflammation. However, the role of IL-17A has not yet been validated in colitis-associated cancer (CAC). This study aims to identify the effects of IL-17A in tumorigenesis utilizing IL-17A-deficient mice in an experimental CAC model. CAC was induced in both the IL-17A-deficient and the C57BL/6 (wild-type, WT) mice by injection of 12.5 mg/kg azoxymethane followed by three rounds of 1.7% dextran sodium sulfate exposure to elicit colitis. On day 63 after the start of the study, mice were sacrificed. Colonic inflammation, proliferation and tumorigenesis were evaluated. Tumor numbers per mouse (1.43 versus 5.80; P = 0.02) and mean tumor size (1.17 versus 3.58 mm; P = 0.01) were significantly decreased in IL-17A-deficient mice compared with WT mice. Furthermore, the inflammation and the proliferation scores of IL-17A-deficient mice were significantly lower than WT mice. In the analysis of inflammatory mediators, IL-6, interferon-γ, tumor necrosis factor-α and IL-17A were markedly decreased in IL-17A-deficient mice compared with WT mice. In the western blot analysis, p-STAT3, cyclin D1, cyclin-dependent kinase 2, cyclin E, Glycogen synthase kinase 3-β and p-Akt were downregulated in IL-17A-deficient mice. Immunohistochemical staining with p-STAT3, Ki-67 and β-catenin revealed lower number of stained cells in IL-17A-deficient mice compared with WT mice. IL-17A ablation significantly decreases CAC tumorigenesis and thus may play an important role associated with chronic colitis.