Background: Chlamydia trachomatis infection rates in Alabama increased dramatically from 1996 to 1998, due in part to expanded screening. Detection of prevalent infection through increased testing could mask timely detection of the impact of screening programs on community prevalence. Goal: The goal of the study was to evaluate the temporal trend in C trachomatis prevalence among women aged 15 to 34 years attending reproductive healthcare clinics in Birmingham, Alabama. Study Design: Between 1995 and 1998, systematic screening of the first 60 women presenting each month to each of six participating clinics was performed with urine-based testing for chlamydia. Results: In contrast to the 218% increase in chlamydia prevalence detected through passive surveillance, during the study period surveillance in sentinel sites revealed that first-visit chlamydia prevalence declined from 12.8% to 8.6%. Conclusions: Sentinel site surveillance served as a valuable adjunct to passive surveillance for determining the true trend of chlamydia prevalence during the hyperendemic phase of a local epidemic.