In order to study the familial aggregation of colorectal cancer we investigated the pedigrees of the patients with adenocarcinoma of the large bowel who underwent a surgical operation between november 1990 and october 1992 at the Istituto di Chirurgia Generale e Sperimentale Of Pisa University. For each proband, information was obtained on his/her four grandparents and all their second generation descendants. The final sample included 99 probands and 1455 relatives. Only two cases with diagnosis of familial adenomatous polyposis were excluded. As a control group, we applied the same methodology to the spouses of probands, collecting in the end a sample of 72 families including 1163 individuals. The frequency of both colorectal and extracolonic cancer was higher in the relatives of cases than in the control group, for all the relationships. Among the first degree relatives, the empirical risk of colorectal cancer was 1/30 among the case families and 1/139 among the control families, for a 4.6 fold increase in risk. For cancers at all sites (colorectal excluded), the corresponding risk were 1/8 and 1/12. We computed the posterior probability of dying from cancer for a random individual, given the known affection status of one or more of his/her relatives of specified relationship. For an individual with one first degree relative affected by colorectal cancer the posterior risk of the same tumor was 1/15, compared to a value of 1/70 for the entire control population. Considering all cancers, colorectal excluded, we obtained the result that for a person with at least three affected relatives, one of first, one of second and one of third degree, the probability of dying from colorectal cancer was 6%. The distribution of the number of affected individuals for kindred was highly skewed, with a few families responsible of a large part of the observed familial aggregation. This was true for both the cases of colorectal cancer and for all-site cancer. However, no family fulfilled the criterion of hereditary nonpolyposis colorectal cancer (Lynch syndromes I or II).