Approximately 5% of all colon (large bowel) cancers are directly caused by inherited genetic abnormalities. These hereditary colon cancers often feature colonic polyps or growths that eventually become cancerous. There are several kinds, including familial adenomatous polyposis (FAP) and a variant called Gardner's syndrome. Another type, hereditary non-polyposis colon cancer (HNPCC), features few if any polyps. Relatively rare hereditary conditions such as Peutz-Jeghers syndrome and juvenile polyposis are not cancer or precancerous conditions but confer on the patient a greater than normal risk of developing colon cancer. Many families with higher than normal rates of colon cancer have one or more of these conditions. While non-hereditary ("sporadic") colon cancer rarely occurs before age 40, hereditary colon cancers often occur in younger people.

Experts in hereditary colon cancers strongly recommend that people with unusually high rates of colon cancers in their families be surveyed regularly for cancerous growths, even if they have no symptoms. Colonoscopy is the most effective and most widely used method of surveillance for hereditary colon cancers. In colonoscopy, a narrow flexible tube is inserted into the colon through the rectum. At the end of the tube is a tiny light and video camera which allows the physician to see the inside of the colon.
Colonic polyps, or adenomas, are relatively easy to diagnose by colonoscopy. FAP and Gardner's syndrome are characterized by hundreds, even thousands, of polyps. If the disease is caught in the precancerous stage, cancer can be prevented by removing the colon surgically. If the disease has progressed to the cancerous stages, surgery may or may not be an effective treatment. In families known to carry a gene for one of these diseases, surveillance for polyps should begin at age 10 to 12.

Although HNPCC usually does not feature the highly conspicuous polyps, it too is best detected with colonoscopy. One type, Lynch syndrome I, begins at a young age, is more likely to be in the proximal (upper, or right) colon, and is often accompanied by other colon tumors. Lynch syndrome II has all of these features and is also often accompanied by cancers outside the colon, especially in the endometrium (the lining of the uterus) and the ovaries. HNPCC may also be associated with cancers of the stomach, small bowel, pancreas, urinary tract, or larynx in some families. In families known to be affected by HNPCC, regular surveillance with colonoscopy should begin at about age 25.

Many clinics, hospitals, and cancer centers that treat people with hereditary colon cancer maintain registries. A registry is a list of families known to be affected by a particular genetic disease. These lists help identify all the family members who might carry the disease and should be surveyed. Registries and regular surveillance help save lives by prevention and early detection of hereditary colon cancers.