To determine prevalence and risk factors for endocervical Chlamydia trachomatis infection in an urban midwestern population and to evaluate two non-culture direct tests for C. trachomatis, we studied 849 women attending two family planning clinics and a community health clinic in Milwaukee, Wisconsin. Adequate endocervical specimens were obtained from 751 women for chlamydial isolation in tissue culture and antigen tests using direct fluorescence (DFA) and enzyme immunoassay (EIA); 93 (12.4%) patients had cultures positive for C. trachomatis. Compared to culture, the DFA test had a 77.4% sensitivity, 96.8% specificity, and a predictive value positive (PVP) of 77%. For the EIA, these values were 83.9%, 97.0%, and 80%, respectively. No single historical, clinical, or laboratory variable, including the previously described cervicitis index and specific cytologic findings on Pap smear, had sufficient predictive value to be used as the only criterion for selective screening in this population. Criteria for selective screening were proposed that would result in screening 43% of patients and would identify 71% of infections. PVP of both non-culture tests was 89% in persons identified by these criteria to be at increased risk of C. trachomatis infection.