5 to 10 percent of colorectal cancers occur due to a genetic predisposition. Familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC) belong to the autosomal-dominant inherited colorectal cancer syndromes. First degree relatives of affected persons are at high risk for colorectal cancer and therefore should undergo regular surveillance programs. The genetic background of FAP and HNPCC is known today, but not in every affected family the germline mutation causing the inherited disorder can be found. In some cases identification of a germline mutation is helpful to confirm a suspected diagnosis in a patient. In almost all cases it is of major importance for their family members, as it makes genetic testing and prediction of cancer risk available for persons at risk. Predictive genetic testing opens up a new dimension in relationship between patients and physicians. Test results are of great importance for future plans of a person at risk, therefore predictive genetic testing should never be performed without genetic counselling. Accordingly the German Bundesärztekammer has worked out guidelines for predictive genetic testing in hereditary tumours.