Background and Objectives: Chlamydial control programs that identify and treat infected persons have been used as a means of reducing prevalence and morbidity. The overall objective of the current study was to examine the prevalence of chlamydial infection during the course of such a program. Goals: To determine whether a reduction in isolation rates was evident among adolescent girls during the 8.75 years that the control program has been in place. Study Design: The study population consisted of all 4,329 sexually active girls between the ages of 13 and 19 attending the four adolescent health clinics in Indianapolis, Indiana, during the period beginning October 1, 1985, and ending June 30, 1994. All girls were cultured for Chlamydia trachomatis, and behavioral data were collected for those attending the clinics before 1989. The trend in quarterly isolation rates was examined using linear regression analysis. Results: Results showed that there was a significant decrease (P = 0.0001), from 25.9% to 9.7%, in the first-visit chlamydial isolation rate over the study period. Behavioral data showed decreases in the frequency of sexual intercourse and in lifetime years of sexual activity, as well as an increase in condom use. Conclusions: The quarterly isolation rates showed that there has been a 63% decline in chlamydial infection among adolescent girls attending the clinics for the first time.