The purpose of this review is to analyze the role of genetic factors in the pathogenesis of human cancer, with particular attention to tumours of the digestive organs. Human neoplasms are defined as "sporadic" when there is no evidence of cancer among relatives besides the index case; "Familial" tumours are characterized by cancer aggregation in a given family, but without verticality or other features of mendelian (autosomal) transmission. In "Hereditary" tumours there is sufficient clinical and biologic evidence to suspect that genetic factors are the main event responsible for their development. Hereditary tumours have been associated with germ-line mutations of oncogenes or, more often, of tumour suppressor genes. More recently, a new category of cancer-related genes has been defined-the mutator genes-which are involved in the mechanisms of DNA repair. Among the various hereditary cancer syndromes, Hereditary non polyposis Colorectal Cancer (HNPCC or Lynch syndrome), Familial Adenomatous Polyposis (FAP) and related syndromes, Hereditary Breast tumours, Li-Fraumeni syndrome and Von Hippel-Lindau disease have been discussed in more detail. Besides purely scientific problems, many ethical and social aspects remain to be solved in hereditary cancer syndromes, and it is likely that their solution will require-in the years to come-a close collaboration between oncologists, geneticists and basic research workers.