Clinical and epidemiologic factors associated with Chlamydia trachomatis infection were examined in women attending two family planning clinics in order to develop criteria for selective screening. Chlamydia trachomatis was isolated from the cervix of 98 (9.3%) of 1,059 women. Five demographic, behavioral, and clinical characteristics were independently predictive of chlamydial infection by stepwise multivariate logistic-regression analysis: aged 24 years or less, intercourse with a new partner within the preceding two months, examination results showing purulent or mucopurulent cervical exudate, bleeding induced by swabbing the endocervical mucosa, and use of no contraception or a nonbarrier method. A screening program that tested women with two or more of these risk factors (65% of the total) would encompass all who had a 4.7% or greater predicted risk of chlamydial infection and would detect 90% of all infections. Selective screening of sexually active women for chlamydial infection is advocated as a necessary and cost-effective public health measure.