BACKGROUND: Screening for colorectal cancer is effective in family members with a high risk of this condition, owing to single gene mutations. However, it is not known which is the most effective method of ascertaining these families at high risk. AIMS: To investigate whether a case-finding approach using computerised general practitioner (GP) registers would improve the ascertainment of families at high risk of colorectal cancer due to family history. DESIGN OF STUDY: Prospective GP registe...