Urine tests for Chlamydia trachomatis permit expansion of screening beyond traditional clinic environments. Prevention of infection in teenagers is a high priority. To define the prevalence of C. trachomatis among teenagers by using the ligase chain reaction assay on urine specimens and to evaluate leukocyte esterase testing of urine specimens as an indicator of infection. Cross-sectional study. An adolescent clinic, a juvenile detention facility, seven school-based clinics, and three community-based youth organizations in Seattle, Washington. 10 118 sexually active teenagers and young adults. Chlamydia trachomatis infection detected in urine specimens by ligase chain reaction assay and leukocyturia detected by leukocyte esterase testing. The prevalence of chlamydial infection among female participants was 8.6% and declined with increasing age; among male participants, it was 5.4% and increased with increasing age. In female participants, independent predictors of infection were being 17 years of age or younger (odds ratio [OR], 1.49), having had two or more sex partners in the previous 2 months (OR, 1.61), and having genitourinary symptoms (OR, 1.46). In male participants, independent predictors were being of nonwhite race or ethnicity (OR, 2.00 to 3.08), having had two or more sex partners in the previous 2 months (OR, 1.57), and having used a condom during the most recent sexual encounter (OR, 0.67). For identifying infection in male participants, the sensitivity of leukocyte esterase testing was 58.9%, the specificity was 94.9%, the positive predictive value was 38.4%, and the negative predictive value was 97.7%. Chlamydial infection is common in teenagers and young adults in community settings. The urine ligase chain reaction assay will permit widespread screening for C. trachomatis, but leukocyte esterase testing had low sensitivity for selecting persons for screening with this assay. Indicators of chlamydial infection differed substantially in male and female participants.