A clinical overview of the familial colonic cancer syndromes has been presented. Those syndromes that are characterized by antecedent colonic polyposis are well known to most clinicians. It has become clear that familial polyposis coli, Gardner's syndrome and the variants of both overlap extensively and represent the multiple clinical faces of a single disease. Research into the underlying mechanism of disease has found abnormalities in the growth characteristics of colonic epithelium in vivo and of cultured skin fibroblasts in vitro. These lines of investigation appear to be leading to a means of making the diagnosis before the first adenoma develops.