About 13% of all colorectal cancer may be dominantly inherited. This amounts to about 300 new cases a year in Norway. Colorectal cancer can be cured by early diagnosis and treatment. Coloscopy with polypectomy may prevent infiltrating cancer. Affected families should be offered genetic evaluation, and family members subjected to regular colonoscopy. The genetic bases of five colorectal cancer syndromes, accounting for most cases of hereditary early onset colorectal cancer, have now been determined. These are familial adenomatous polyposis, colon-endometrial cancer (hereditary non-polyposis colon cancer), Cowden's syndrome, Peutz-Jegher's syndrome and juvenile polyposis. These account for at most 3% of all colorectal cancers. In this group, predictive genetic testing may be employed in families with known mutation. Demonstration of mutation carriers by predictive testing must be based on health service available to the persons at risk. With regard to prophylactic measures, experimental and epidemiological data suggest a preventive effect of aspirin and resistant starch. Empirical information on the effect of intervention is insufficient; multicentre studies are needed.