Abstract: Objective To investigate the expression of interleukin-9 (IL-9) in colon cancer tissues and its clinical significance. Methods Immunohistochenmistry and qRT-PCR were used to detect the expressions of IL-9 protein and mRNA in 92 colon cancer tissues and paired adjacent normal tissues. The correlation of IL-9 expressions with the clinicopathological features and prognosis of the patients was analyzed. Results IL-9 protein and mRNA expressions were significantly higher in adjacent normal tissues than in the colon cancer tissues (P<0.001). In colon cancer patients, IL-9 expression was significantly correlated with TNM stage (P=0.013), Ducks stage (P=0.025) and lymph node metastasis (P=0.004) but not with gender, age, tumor size, differentiation or hepatic metastasis (P>0.05). The survival time of colon cancer patients with positive IL-9 expression was significantly longer than that of patients negative for IL-9 expression (P=0.015). Conclusions IL-9 expression is lowered in colon cancer tissues compoved with in the adjacent normal tissues. IL-9 expression is negatively correlated with TNM staging, Ducks staging and lymph node metastasis but positively with good prognosis, suggesting its important role in the tumor microenvironment of colon cancer.