A case-control study was undertaken of the family histories of colorectal cancer in 128 patients with colorectal cancers and those of 61 patients with colorectal adenomas and matched surgical control patients who were attending a regional surgical service in Western Australia. One family with multiple polyposis of the colon was excluded from the study. A history of colorectal cancer in one or more first-degree relatives was associated with a relative risk of colorectal cancer of 2.5 (95% confidence interval, 0.8 to 8.0), of adenoma of 2.0 (95% confidence interval, 0.5 to 8.0) and of any colorectal neoplasm of 2.3 (95% confidence interval, 0.9 to 5.6). Four patients with colorectal cancer and one patient with colorectal adenoma had more than one first-degree relative with colorectal cancer, whereas no control subject gave this history. The five families that were represented by these cases each showed some other features of non-polyposis familial colorectal cancer. It was estimated that familial factors could explain 60% of colorectal cancer in persons with a family history of the disease in a first-degree relative and 5% of colorectal cancer in the population as a whole. Haemoccult II tests were posted to 629 living first-degree relatives of the patients with colorectal cancers and adenomas; 44% of these relatives returned the completed tests. Four relatives with positive results of tests both before and after dietary restriction were investigated; all four subjects had colorectal adenomas. In addition, one subject had a short segment of ulcerative colitis. A further mailing of Haemoccult II tests one year later gave a 39% response rate; no further cases of colorectal neoplasia were found. One relative developed carcinoma of the caecum 10 months after a negative result in the first round of Haemoccult screening. Persons with two or more first-degree relatives with colorectal cancer, with or without other features of non-polyposis familial colorectal cancer, are at a high risk of the development of colorectal cancer. The comparatively-poor response to an offer of Haemoccult II testing and its known insensitivity and lack of specificity suggest that it is not a satisfactory method of screening these high-risk subjects.