&NA;Close relatives of patients with colorectal cancer are at an increased risk of developing a colorectal malignancy themselves.PURPOSE:A study was conducted to compare risks in relatives of patients diagnosed at different ages.METHODS:Family histories were taken from two cohorts of patients with colorectal cancer: Group A, a population group of 65 patients diagnosed at or under 45 (median, 42) years; Group B, 212 patients of all ages (median, 68 years) treated in a single surgeon's practice.RESULTS:Overall relative risk of colorectal cancer in first‐degree relatives was 5.2 in Group A and 2.3 in Group B. There was familial clustering of colorectal cancers suggestive of hereditary nonpolyposis colorectal cancer in 13 (20 percent) families to Group A but to only 3 (1.5 percent) families in the second group. Cumulative incidence of colorectal cancer for relatives of the young cohort rose steeply from 40 years, reaching 5 percent at age 50 years and 10 percent at age 70 years. This contrasts with risk for relatives of older patients, in whom the shape of the curve resembles that of the overall population risk, reaching 5 percent at age 70 years and 10 percent at age 80 years.CONCLUSIONS:There appears to be a quantitative and qualitative increase in risk to relatives of patients diagnosed at a young age compared with those diagnosed later to life, at least part of which is likely to be the result of a hereditary susceptibility. Close relatives of early onset cases warrant more intensive endoscopic screening and at an earlier age than relatives of patients diagnosed at older ages.