Epidemiologic studies have shown that asymptomatic adult relatives of colorectal cancer patients are at increased risk for developing this tumor. A prospective, published pilot study confirmed this added risk and demonstrated the importance of the family history of cancer as a marker of potential malignancy. The study group was enlarged to include 471 asymptomatic adult, first degree relatives of patients having large bowel neoplasia (cancer or adenomatous polyps) but without polyposis syndromes. These first degree relatives were screened by fecal occult blood examinations and flexible sigmoidoscopy, followed by colonoscopy when indicated. Adenomatous polyps or cancer were found in 8.1% of the study group as compared with 3.7% in a comparison group of screens, not having the same family history of neoplasia and undergoing similar screening tests. Of the study group the age-adjusted rate for colorectal adenomas or cancer increased threefold (P less than 0.001) for subjects older than 40 years and an even higher fivefold relative risk was found for large bowel cancer only (P = 0.01). This was true even if there was only one relative with colorectal neoplasia (P less than 0.01) but was even more pronounced among those having more than one affected relative. The results confirm the usefulness of the family history, of even one member with large bowel neoplasia, in isolating a group at high risk for these lesions. This group would most likely benefit from regular cancer and adenomatous polyp screening particularly when older than 40 years.