Juvenile detention facilities house adolescents at high risk for sexually transmitted diseases. Collaboration between health departments and juvenile detention authorities can provide routine, cost-efficient chlamydia screening and treatment to females with limited access to care. We describe trends in screening, positivity, treatment, and associated costs in a well-established juvenile detention chlamydia screening program. In the California Chlamydia Screening Project, juvenile detention facilities in 12 counties collected quarterly aggregate data on female census and line-listed chlamydia test results and treatment data from fiscal year (FY) 2003-2004 to FY 2013-2014. Trends in the proportion of females screened, positivity, and treatment by age, race/ethnicity, and facility volume were evaluated by Cochran-Armitage test. The median cost of the program per chlamydia positive identified was compared by facility in FY 2013-2014. Data from 59,518 test records among juvenile females indicated high screening rates (75.1%-79.4%). Chlamydia positivity, although consistently high, decreased from 14.8% in 2003-2004 to 11.5% in 2013-2014 (P < 0.001). Documented treatment decreased (88.8% in 2005-2006 to 79.0% in 2013-2014, P < 0.001); of those treated, treatment within 7 days increased (80.1% in 2005-2006 to 88.8% in 2013-2014, P < 0.001). The median cost per chlamydia positive identified was $708 (interquartile range, $669-$894) and was lowest for facilities with high chlamydia positivity. The California Chlamydia Screening Project demonstrated consistently high rates of chlamydia screening and positivity among adolescent females while keeping costs low for high-volume facilities. Further improvement in timely treatment rates remains a challenge for extending the impact of screening in this high-risk population.