To assess the familial risk of colorectal cancer among first-degree relatives of colorectal cancer patients, a tumour-related family history study was conducted in 1982-1988 on 184 patients (median age 60 [28-92] years). These family data were compared with the data recorded in the cancer register of the population of Basel. An enhanced risk among first-degree relatives was noted (relative risk: 1.8%; 95% confidence interval: 1.4-2.3). The enhanced risk was attributed exclusively to seven families involving 2-3 relatives each (the relative risk in these families being 34). In four of these seven families an autosomal dominant site-specific cancer of the colon (Lynch's syndrome I) was diagnosed. Hence, family case history of colorectal cancer patients enables the identification of relatives with a high risk for colorectal carcinoma.

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