When considering the impact of a genetic diagnosis of hereditary predisposition to colon cancer, there are many similarities to other predictive genetic tests, but also many differences. The development and availability of such genetic diagnoses, and the concept of testing being linked to effective prevention, have advanced rapidly, opening up not only unique opportunities but also unique psychosocial situations for the affected families-and unusual ethical issues for the professional. Compared to a diagnosis of sporadic colorectal cancer for a patient, hereditary colorectal cancer requires an understanding of genetics, heredity, and the attendant mathematics of risk calculation, but, most importantly, there must be a belief that it is possible to remain healthy whilst having an increased risk. This paper outlines the possible impact of a genetic diagnosis of hereditary non-polyposis colorectal cancer (HNPCC) or familial adenomatous polyposis (FAP) on both the individual and the family and concludes that genetic testing should be accompanied by genetic counseling. Relevant ethical issues are also introduced, with the opinion presented suggesting that if primary considerations are always for the individual rather than the family or society, then unethical or eugenic decisions are likely to be avoided.