In two rural and two urban family planning clinics in central Pennsylvania, the authors screened 889 women making routine visits for Chlamydia trachomatis using the Chlamydiazyme immunoassay method (Abbott Laboratories; North Chicago, IL). C. trachomatis antigens were detected in 11.2% of the women; they found no differences in prevalence between rural and urban clinics. Among 21 clinical characteristics and seven risk factors, younger age (younger than 25), oral contraceptive use, gonococcal infection, mucopurulent exudate, abnormal vaginal discharge, and cervical ectropion were associated with chlamydial infection. Logistic regression revealed that age alone was independently associated with infection. The authors found that screening criteria derived from other epidemiologic studies generally did not predict the presence of C. trachomatis in the present sample. In a follow-up study performed at least one year later, 169 patients were re-screened; 47 tested positive and 122 negatives for C. trachomatis. Overall, 11.8% were infected, which was 12.4% of those who originally tested negative and 10.6% of those who originally tested positive.