Population screening for cystic fibrosis (CF) carriers, now possible because of the cloning of the CFTR gene, merits evaluation because CF is common, serious, and without satisfactory treatment, and because prenatal diagnosis is available. Clinical trials of CF population carrier screening are reviewed. These trials have involved pregnant women, adults of both sexes of reproductive age, or adolescents. Schools, the usual setting for screening programs for adolescents, provide an excellent opportunity for a formal educational component and for comprehensive coverage of the population, but compared to a health-care setting, may entail subtle coercion and may compromise confidentiality. In the case of adults, many say they prefer screening before conception but do not see a physician for evaluation before conception and providers find screening more readily accomplished in the setting of a prenatal visit. Two large U.S. studies of prenatal screening with quite different subject populations and health-care settings encountered few of the adverse outcomes originally predicted for CF carrier population screening.