Colorectal cancer (CRC) develops from normal epithelium, through dysplastic adenoma to invasive carcinoma. In addition to familial adenomatous polyposis and Lynch syndrome, approximately 10-35% of CRCs are familial in nature. CRC screening and surveillance programmes are based on an understanding of the natural history of polyps and rely on the ability to remove premalignant lesions endoscopically before they are capable of developing invasion. There are, however, significant differences in these guidelines between the UK and the USA in relation to the weight attributed to a family history of polyps. Here, using publicly available national data sets, we show that these differences in guidelines unexpectedly generate inadequate screening recommendations for second-degree relatives of patients with CRC in the UK. We validate our simple mathematical modelling of the clinical problem on a regional data set as well as previously published study data to demonstrate the correct interpretation. We further discuss the implications of a family history of adenoma formation in the current climate of the Bowel Cancer Screening Programme and suggest a re-evaluation of the UK guidelines in the light of this developing issue.