The frequency of colonic adenomatous polyps and the incidence of colorectal cancer in close relatives were evaluated in a prospective study performed in 100 consecutive patients operated on for colorectal cancer. One hundred patients matched for age and sex, in whom double contrast enema and colonoscopy failed to show cancer, served as control group. Colorectal carcinomas in first-degree relatives were found in 11% of the surgically treated patients and 6% of the control group (the difference is not statistically significant). Solitary or discrete adenomas in patients operated on for colorectal carcinomas were significantly more frequent (32%) than in the control group (18%) (P less than 0.05). This difference is also statistically significant when considering only those patients without relatives suffering from carcinoma; however, the same cannot be statistically proven with the small group of patients with a positive family history. Present findings do not indicate that single or discrete adenomas synchronous with colorectal cancer are significantly associated with a familial history of large bowel malignancy. These findings are consistent with the hypothesis of environmental factors being involved in adenoma pathogenesis.