In colon cancer,classic disease staging remains the key prognosis and treatment determinant.Although adjuvant chemotherapy has an established role in stageⅢcolon cancer patients,in stageⅡit is still a subject of controversy due to its restriction to a small subgroup of patients with high-risk histopathologic features.Patients with stageⅡtumors form a highly heterogeneous group,with five-year relative overall survival rates ranging from 87.5%(ⅡA)to 58.4%(ⅡC).Identifying those for whom adjuvant chemotherapy would be appropriate and necessary has been challenging,and prognostic markers which could serve in the selection of patients more likely to recur or benefit from adjuvant chemotherapy are eagerly needed.The stronger candidate in this category seems to be microsatellite instability(MSI).The recently reported European Society for Medical Oncology guidelines suggest that MSI should be evaluated in stageⅡcolorectal cancer patients in order to contribute in treatment decisionmaking regarding chemotherapy administration.Thehypothetical predictive role of MSI regarding its response to 5-fluorouracil-based adjuvant chemotherapy has proven a much more difficult issue to address.Almost every possible relation between MSI and chemotherapy outcome has been described in the adjuvant colon cancer setting in the international literature,and the matter is far from being settled.In this current report we critically evaluate the prognostic and predictive impact of MSI status in patients with stageⅡand stageⅢcolon cancer patients.