Antibody-based methods for the diagnosis of Chlamydia trachomatis infection of the cervix have recently made population screening programs possible for this epidemic and frequently asymptomatic problem. We constructed a decision model, using medical care costs as utilities, to determine the total costs of screening and of not screening in California state-funded family planning clinics, and to determine the prevalence of infection at which such screening could be expected to pay for itself. A net savings of $6 million would be realized in the first year, with annual savings eventually increasing to over $13 million, from the prevention of chlamydia-associated pelvic inflammatory disease and other long-term sequelae such as tubal infertility and ectopic pregnancy. Over $60 million could be saved in the first five years of such a statewide screening program. In populations with infection prevalence of 2% or more, such screening will pay for itself and can be considered "cost-effective." Screening of asymptomatic women for chlamydia should be carried out in most American family planning clinics.