The knowledge acquired in genetics and molecular biology over the last 2 decades has led to advances in the molecular diagnosis of some diseases, among them hereditary forms of colorectal cancer such as hereditary non-polyposis colorectal cancer and familial adenomatous polyposis. Moreover, the discovery of the genes causing these diseases has had implications beyond hereditary diseases since the same genes that cause hereditary forms of cancer also play a role in the much more frequent sporadic forms. Genetic diagnosis allows clinical diagnosis to be confirmed, as well as presymptomatic and even prenatal diagnoses to be made, with implications for patients with these hereditary diseases and their families.