Microsatellite instability (MSI) phenotype occurs in approximately 15 to 24% of colorectal cancer (CRC) patients and may be sporadic or hereditary. It reflects a mutator phenotype in the tumor due to a lack of mismatch repair system. MSI is indeed one of the characteristics of CRCs occurring in Lynch syndrome and some sporadic cases. CRCs with MSI have a better prognosis than CRCs with microsatellite stability (MSS). This is explained partly by a more important anti-tumor immune response and by apoptosis of tumor cells in which mutations accumulate. However, in some retrospective studies, microsatellite instability in stage II CRCs was associated with no benefit to or even a deleterious effect of 5-FU alone based adjuvant therapy. Nevertheless, results obtained in stage III CRCs with FOLFOX type adjuvant chemotherapy remain favorable in retrospective studies.