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The familial occurrence of tumors has been investigated in 389 subjects with colorectal cancer by reviewing the clinical data and the genealogical tree of all patients registered in 1984-1986, in the Local Health District, for malignancies of the large bowel. Among first-degree relatives of the registered patients there were 89 cases of colorectal cancer as opposed to 19 in a hospital-based control group matched for age and sex [odds ratio (OR), 7.5, P less than 0.001]. This excess of neoplasms among relatives was particularly evident in siblings (60 versus 7, OR 14.7, P less than 0.001) but it was observed also in parents (27 versus 12, OR 4.2, P less than 0.01). Besides colorectal cancer there was no significant excess of other types of tumor in case families, whereas lung tumors tended to be more frequent in control relatives (32 versus 17). Almost half of the registered patients (182 out of 389) had one or more cases of cancer of any sites among relatives; similarly, in 68 there were one or more relatives affected by (or deceased for) colorectal cancer. Moreover, in 27 patients (7.0%) there were at least three cancers of any sites among relatives and in 15 the excess (two or more) was limited to neoplasms of the large bowel. In patients without or with only one neoplasm among relatives, cancers were mainly located in the left colon; however, cancer of the right colon became relatively more frequent in patients with two or more tumors in close relatives. In conclusion, the present study suggests that in approximately 15-20% of patients registered for colorectal cancer one or more first-degree relatives are affected by neoplasms of the large bowel. This familial occurrence of intestinal malignancies (but not of tumors of other organs) strongly suggests a genetic susceptibility to colorectal cancer in a fraction of these patients. Moreover, in a further subgroup of individuals (approximately 5% of all cases) the familial aggregation of two or more cases of colorectal cancer among relatives (besides the proband) and the frequent location of tumors in the right colon make the diagnosis of Lynch syndrome extremely probable.