Evidence is mounting that colorectal adenocarcinoma can be prevented by detecting and removing adenomatous polyps, and that detecting early-stage cancers reduces mortality from the disease. Both polyps and early-stage cancers are usually asymptomatic; cancers that have grown large enough to cause symptoms have a much worse prognosis. Most people will be of average risk and require screening for colorectal cancer and polyps beginning at age 50. However, a substantial number of people are at increased risk because of an inherited predisposition to the disease and need screening or treatment as early as puberty. Germline and somatic truncating mutations of the adenomatous polyposis coli gene are thought to initiate colorectal tumor formation in familial adenomatous polyposis (FAP) and sporadic colorectal carcinogenesis, respectively. Genetic testing for FAP can help guide surveillance and treatment of patients at risk for the disease. Hereditary nonpolyposis colorectal cancer (HNPCC) is thought to be the result of DNA mismatch repair deficiency, and genetic testing for HNPCC may ultimately prove to have clinical value for patients in HNPCC families.