Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominant condition caused by highly penetrant gene mutations. It is characterized by a higher susceptibility to colorectal cancer and others, especially endometrial tumors. This syndrome is originated from the inheritance of mutations in DNA mismatch repair genes (MMR). The most commonly affected genes in HNPCC are MLH1 and MSH2 and its deficient translation renders the cell susceptible to the accumulation of many molecular defects, a condition which can be evaluated by the microsatellite instability, the so-called MSI phenotype. Nowadays the early detection of HNPCC is possible. Genetic tests to investigate the syndrome are available and can be effective for the precise diagnosis of HNPCC in at least 70% of cases. Moreover, these tests allow characterizing which individual within a family carries the mutated gene. There are, however, financial and psychological implications that hamper their realization in every person that presents colorectal cancer. Due to this fact, some clinical criteria were developed by a HNPCC international committee to select families with high probability to bear the mutation. The aim of this study is, based in those criteria and in the advances of genetic testing, to orientate the physician about his patient’s surveillance, suggesting in which situation a genetic test is warranted.