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. 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. Prospective GP register study. General practices in Oxfordshire. Identification of patients with colorectal cancer using GP registers, followed by a family history questionnaire survey to identify those at high risk. Using GP registers, 758 patients with a diagnosis of colorectal cancer were identified; a prevalence of 172 cases per 100 000 (95% confidence interval = 159 to 184). Of these, 305 patients, diagnosed under the age of 65 years, were sent a family history questionnaire. Two hundred and one (66%) patients responded to the survey; 10 (5%) patients were assessed as having high-risk families and 47 (23%) patients were assessed as having families at moderate risk. Eight of the high-risk patients had 34 first degree relatives who would benefit from routine disease surveillance, and 37 moderate-risk patients had 153 first degree relatives. Only two high-risk and six moderate-risk patients identified were previously known to the local Clinical Genetics Department. A case-finding approach using GP records and a family history questionnaire is an effective way of identifying families at high risk of developing colorectal cancer, who can then be offered disease surveillance.