Hereditary non-polyposis colorectal cancer (HNPCC) probably constitutes 5% of all the cases of sporadic colorectal cancer. At present, the diagnosis can only be established on the basis of a family history which should fulfill the "Amsterdam criteria": 1) Colorectal cancer in at least three family members, 2) One family member must be a close relative of the other two, and 3) The diagnosis must have been established prior to the age of 50 years in at least one relative. Other forms of cancer also occur in the HNPCC syndrome, particularly endometrial cancer. The syndrome has a dominant inheritance and, therefore, all close relatives should be submitted to control examinations for the most important forms of cancer associated with the syndrome.