Objective. The aim of the study was to estimate the prevalence of hereditary cancers and the need for surveillance in Telemark county, Norway. Material and methods. All persons attending the Norwegian Colorectal Cancer Prevention (NORCCAP) trial in Telemark were interviewed about cases of cancer in the family. Diagnoses were verified, pedigrees constructed and families classified according to preset criteria aiming at identifying hereditary cancer. Mutation analyses were performed in kindreds at risk for breast cancers when possible. Immunohistochemistry of tumors in assumed inherited colorectal cancer families was undertaken. Results. The screening examination was attended by 7224 persons among whom 2866 had cancer in the family. Of these, 2479 had no suspicion of any known inherited cancer syndrome. Family information questionnaires were mailed to 387 persons and returned by 191. Sixty-four of these 191 met the clinical criteria for familial cancer by family history after verification of diagnoses. Observed prevalences for being at risk for hereditary breast and breast-ovarian cancer (HBOC) or hereditary non-polyposis colorectal cancer (HNPCC) were 2.8‰and 0.77‰, respectively. Conclusions. The number of colonos-copies and mammograms obtained per year serving those who needed them was limited and reduced by clinical genetic work-up from 2866 with a family history of cancer to 64 proven cases. Continued surveillance of an unnecessarily high number leads to unjustified cancer worry, is costly and uses up health-care facilities. Genetic work-up is a one-time job that reduces input numbers to surveillance programs, provides a starting-point for mutation testing and is economically cost beneficial if inherited cancers are prevented or cured by the health-care programs offered. Objective. The aim of the study was to estimate the prevalence of hereditary cancers and the need for surveillance in Telemark county, Norway. Material and methods. All persons attending the Norwegian Colorectal Cancer Prevention (NORCCAP) trial in Telemark were interviewed about cases of cancer. In the family. Diagnoses were verified, pedigrees constructed and narrow taxonomic staff. . The screening of the witness was was represented by 7224 persons among whom 2866 had cancer in the family. Of these, 2479 had no suspicion of any known inherited cancer syndrome. Family information questionnaires were mailed to 387 persons and returned by 191. Sixty-four of these 191 met the clinical criteria for familial cancer by family history after verificatio n of diagnoses. Observed prevalences for being at risk for hereditary breast and breast-ovarian cancer (HBOC) or hereditary non-polyposis colorectal cancer (HNPCC) were 2.8‰and 0.77‰, respectively. Conclusions. The number of colonos-copies and mammograms Obtained per year serving those who needed them was limited and clinical genetic work-up from 2866 with a family history of cancer to 64 proven cases. Continued surveillance of an unnecessarily high number leads to unjustified cancer worry, is costly and uses up health -care facilities. Genetic work-up is a one-time job that improves input numbers to surveillance programs, provides a starting-point for mutation testing and is economically cost beneficial if inherited cancers are prevented or cured by the health-care programs offered.