To investigate the familial aggregation of colorectal cancer in Denmark, parents and siblings of colorectal cancer patients diagnosed below age 60 years in the years 1982-1984 were identified through population registries. For 1,470 probands with families eligible for tracing, 1,376 mothers, 1,303 fathers and 3,259 siblings were identified. They contributed 222,634 person-years, and 325 cases of colorectal cancer were observed during the follow-up period 1943-1992. All data were retrieved from population registries and consequently were free from any reporting bias. The overall standardized morbidity ratio (SMR) compared with the Danish population was 2.02 (95% confidence interval [CI] 1.81-2.25), significantly different between the parents (1.78, 95% CI 1.55-2.04) and the siblings (2.65, 95% CI 2.21-3.17). A strong dependence on the proband's age at diagnosis was seen for the sibling risk; siblings of probands less than 50 years old at diagnosis had a 5-fold risk compared with the general population. This dependence was not seen for parents, but the risk tended to be higher for parents of younger ages. No other factor was seen to influence the relative risk. The observation of an 80% increased risk among the parents and a 170% increased risk among the siblings indicates that the genetic component is one source, but probably not the only one, of familial aggregation of colorectal cancer. The cost benefit of screening siblings of colorectal cancer patients is substantially higher than that for the total population.