Abstract: Objective : To explore the expression of indoleamine 2, 3-dioxygenease (IDO) in colon carcinomas and regional lymph nodes, as well as its clinical significance. Methods : Expression of IDO was detected with immunochemistry in 110 cases of colon carcinoma including primary tumor, adjacent normal tissue, and regional lymph nodes. Meanwhile, the relationship between IDO expression and clinicopathological characteristics of the patients was analyzed. Results ； IDO expression in primary tu-mors was similar to that in adjacent normal tissue. A relationship between IDO expression and gender, age, TNM stage and 5-year survival was not observed. However, IDO was expressed in dendritic-like cells in regional lymph nodes without metastasis，and higher expression of IDO in these regional lymph nodes was correlated with poorer 5-year survival (31.8%). Conclusion : Before metastasis IDO induces immunological suppression in regional lymph nodes, which may prepare the microenvironment for metastasis. However, the IDO expression in tumor cells the mselves seems to be uninvolved in this pro-