We searched for colorectal tumors in asymptomatic patients older than 40 years with family history of sporadic colorectal cancer (only first degree relatives). One hundred and four patients at risk had a left-sided (n = 60) or a total colonoscopy (n = 44) and were compared to 104 control patients, matched for age, sex and type of colonic investigation. Three cancers were detected in the group at risk, 1 in the control group (NS). One or more adenomas without carcinoma were found in 10 percent of the patients at risk and in 9 percent of the controls (NS). There was no difference between groups in the number, size, histologic type, degree of dysplasia, or location of adenoma in the colon. These results do not exclude a family predisposition to sporadic colorectal cancer or adenoma. Because of the low rate of adenoma detection, relatives of patients with sporadic colorectal carcinoma but without personal risk factors cannot be considered as a high risk group for colorectal endoscopic screening.