The sample consisted of 5,558 adolescents processed through the Cook County (Illinois) Juvenile Temporary Detention Center from April 13, 1998, to July 13, 1999. Results from the LCR urine testing for both gonorrhea and chlamydia were recorded, along with a unique patient identifier and demographic information. The patient identifier was used to examine reinfection rates of those adolescents who were processed through the facility on more than one occasion during the study period. Prevalence rates were calculated by a positive LCR test result at any screening (percentage positive). Reinfection rates were calculated for adolescents who initially tested positive for either gonorrhea or chlamydia and received a subsequent screening more than 30 days later. In addition, subsequent screening results were examined using data from all adolescents who received a second screening more than 30 days after the initial screening, regardless of their initial test result. Overall, the prevalence was 5.1 percent for gonorrhea and 14.7 percent for chlamydia. Female adolescents were 3.5 and 3.3 times more likely to have gonorrhea and chlamydia, respectively, than were male adolescents. Reinfection rates for the 180 adolescents who had a sexually transmitted disease (STD) at first screening and were screened on another occasion were 10 percent for gonorrhea and 28.9 percent for chlamydia. Given the high STD prevalence and reinfection rates uncovered in this study, administrators at juvenile detention facilities could potentially decrease the long-term cost burden on their facilities through a screening program designed to detect STD's before the detainees experience the costly sequelae of STD's or are released into the community to further spread the STD's. Research is also needed to devise intervention strategies that are effective in reducing risky sexual behaviors and STD morbidity in this high-risk adolescent population. 2 tables and 26 references