Adenomatous polyposis coli is an autosomal dominant disease characterized by the development of hundreds of colorectal adenomas in young adults. If prophylactic colectomy is not performed, colorectal cancer develops in virtually all affected individuals by the fifth decade of life. All at-risk relatives older than 10 years of age need to be screened regularly by endoscopy. Recently, the gene responsible for the disease, the APC gene, was cloned. The finding of inactivating mutations of the APC gene in Italian APC patients allowed us to offer DNA-based diagnostic tests to these families.