Pathologic reviews and clinical studies demonstrate that groups with increased cancer risk can be identified. It is estimated that about 3.5 per cent of colorectal cancers in this country are the result of known heritable cancer syndromes, such as familial polyposis. Much effort is currently being devoted to evaluation of biologic markers, such as cell surface antigens and their antibodies, ornithine decarboxylase, errors in DNA repair, abnormalities in metabolism of polyadenosine diphosphate, and application of molecular genetic techniques to identify patients with genetic cancer susceptibility.