Familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC) are two syndromes of colorectal cancer predisposition, inherited in an autosomal dominant fashion. They account for about 1% and 5-7% of all colorectal cancer cases, respectively. FAP is caused by germline mutations of a tumour suppressor gene, the adenomatous polyposis coli (APC) gene, whereas HNPCC results from genetic alterations of the DNA mismatch repair genes. Clinical manifestations in FAP include colonic as well as extracolonic sites (duodenum, eye, dental, nervous or connective tissues). In FAP, prophylactic colectomy is required in all affected patients and regular endoscopic check-up of the upper gastrointestinal tract is necessary to detect malignant transformation of duodenal polyps; medical management of complex desmoid tumours is preferred rather than surgery. In HNPCC, there are extracolonic associated endometrial, gastric, small bowel or brain carcinomas. At present time, for HNPCC patients, only preventive measures such as regular colonoscopic or gynecologic examinations are recommended, since prophylactic colectomy or hysterectomy are not considered to be routine procedures.