Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal, dominantly inherited disease leading to a marked increase in cancer susceptibility, notably colorectal cancer, affecting up to one in 400 individuals in the Western world. Four genes responsible for the majority of cases have been identified. Colorectal cancer in affected people tends to be right sided, occur at an earlier age, and there is a propensity for synchronous or metachronous lesions. Extra-colonic tumours may occur with an elevated frequency, most importantly cancer of the endometrium, but also stomach, hepatobiliary system, small bowel, proximal ureter and renal pelvis, and ovary. On account of these features, management guidelines for members of HNPCC kindreds require modification from those generally advised for patients with sporadic tumours. The cardinal feature for the identification of affected families is the family history. All clinicians have a duty to identify such patients under their care as appropriate screening and surgery should lead to an improved prognosis for such patients and their families.