Screening of patients with a positive family history of colorectal cancer.

A positive family history is one of the most common risk factors for colon cancer. The risk varies from moderately increased when a first-degree relative has colon cancer to high when a first-degree relative is diagnosed at an age less than 50 years or there are two first-degree relatives with large bowel malignancy. The risk of colon cancer is extreme in the rare colon cancer syndromes of familial adenomatous polyposis and hereditary nonpolyposis colon cancer. The extensive characterization of familial occurrence of colon cancer has led to specific screening strategies for each of these levels of familial risk.