Family histories were obtained from 209 patients undergoing treatment for cancer of the colon or rectum at St Mark's Hospital. Death certificates were obtained for parents and siblings who had died. Among these, the number of deaths from cancer of the bowel was more than three times that expected for a comparable group from the general population. There are three principal features which, if present in an index case, are associated with the likelihood that other members of the family will also have had cancer of the large bowel. These are, first, the presence of multiple benign or malignant tumours in the large bowel, secondly, a history of previous primary carcinoma or adenoma of the large bowel and, thirdly, early age of onset of carcinoma of the bowel.