Population carrier screening for cystic fibrosis (CF) was offered to all patients aged 16-45 in one general practice in South Wales, excluding those in couples with a current pregnancy. Out of 1553 patients in this group, 481 subjects were tested, giving an overall uptake rate of more than 30%. The rate of uptake varied with the mode of invitation. Twenty-six carriers were identified, giving a prevalence of identified carriers of 5.4% (1 in 18.5) for those with no family history of CF. A further 18 carriers were identified by cascade testing of these 26. We describe the practical difficulties encountered in setting up this programme in primary care in South Wales. Questionnaires were administered or distributed to all subjects before and after testing. The response rate for the pre-test questionnaire was 95%, and 40-50% for the post-test questionnaires. These showed that, at 3 months post-test, 1 in 4 screen-negative subjects did not appreciate that they had a residual risk of being a carrier. At the same time, 15% of this group thought that there was a 1 in 4 chance of a child being affected if one parent was screen-positive (carried an identified mutation) and the other was screen-negative, and 40% thought there was no risk. Anxiety in relation to testing did not appear to be a major problem, although individual patterns of response to carrier status varied widely and more sensitive indicators of psychosocial impact of genetic tests are required. A pilot study of couple screening showed that this approach is unlikely to be useful in primary care, although we did not assess couple testing during pregnancy. For any programme of CF carrier screening to be established in primary care, it will be necessary to involve the primary care team from the earliest planning stage, so that the opportunity costs, training needs and other costs of the programme can be fully resourced.