Colorectal cancers (CRCs) with microsatellite instability (MSI) are due to a defect in the DNA mismatch repair (MMR) system resulting in an accumulation of frame-shift mutations. They are characterized by a tumor microenvironment richer in cytotoxic CD8 T-cells (CTLs) and a better prognosis compared to microsatellite stable (MSS) CRCs. The mechanisms by which defective MMR system may influence tumor-infiltrating immune cells and their impact on patient survival were still unclear. Thus, we performed a comprehensive analysis of MSI colorectal tumors. We found that the numbers of frame-shift mutations potentially resulting in neo-epitopes were positively correlated to the density of tumor infiltrating CD8 T-cells but were lower than expected at random. We also evidenced that MSI patients could naturally harbor CTLs targeting frame-shift mutation-derived antigens. This favors the hypothesis of an active immunosurveillance in MSI colorectal tumors leading to the genetic evidence of an immunoediting. To evaluate the link between MSI tumor immune contexture and prognosis, we took advantage of a standardized assay that we developed to quantify tumor-infiltrating T-cells, the Immunoscore. Multivariate analyses revealed an advantage of Immunoscore over MSI in predicting recurrence and survival. Our data suggests that the prognostic value of MSI could be attributed to major underlying differences of infiltrating immune cells. Immunotherapeutic treatments, that are more efficient in patients with a preexisting anti-tumor immunity, were approved in MSI patients following successful clinical trials. We suggest that the Immunoscore could be used not only for colorectal tumor prognosis but also for predicting responses to immunotherapies.