The Lynch syndrome [Hereditary Nonpolyposis Colorectal Cancer (HNPCC)] is a dominantly inherited syndrome characterized by the development of a variety of cancers including cancer of colorectum, endometrium, and less frequently, cancer of the small bowel, stomach, urinary tract, ovaries, and brain. The syndrome is due to a mutation in one of the DNA-mismatch repair (MMR) genes. The main features of the syndrome are an early age of onset and the occurrence of multiple tumors. Knowledge of the specific features of the syndrome is crucial for the identification of the Lynch syndrome. In previous years, the Amsterdam criteria were used for recognition of the syndrome. Since we know that the Lynch syndrome is caused by an MMR defect and that the hallmark of the syndrome is microsatellite instability (MSI), more attention should be given to the so-called Bethesda guidelines. These guidelines describe practically all clinical conditions in which there is suspicion of the Lynch syndrome and in which a search for MSI is indicated.