OBJECTIVES: This study aimed to determine the prevalence of sexually transmitted infections (STIs) in symptomatic adolescent female patients presenting to a pediatric emergency department (ED). Secondary objectives included determining correlates of infection. METHODS: This was a prospective prevalence study of a consecutive sample of female patients aged 14 to 19 years presenting to a pediatric ED with lower-abdominal or genitourinary complaints between August 2009 and January 2010. Patients were tested for Neisseria gonorrhoeae, Chlamydia trachomatis, and Trichomonas vaginalis. Descriptive statistics, sensitivity analysis to account for untested patients, and logistic regression to understand correlates of STI were performed. RESULTS: A total of 276 patients met inclusion criteria; 236 underwent STI testing. The prevalence of any STI was 26.3% (95% confidence interval [CI], 20.6%-31.9%) among patients who had testing performed, with C. trachomatis infection being most prevalent (19.7%; 95% CI, 14.5%-24.9%). Assuming all eligible patients who did not undergo STI testing were not infected, sensitivity analysis revealed a minimum STI prevalence of 22.5% (95% CI, 17.5%-27.4%). Multivariable logistic regression revealed no significant association between STI and patient age, chief complaint, or insurance status. There was a significant association between STI and black or African-American race (odds ratio, 9.5; 95% CI, 2.1-44.1). CONCLUSIONS: A large percentage of our study population had an STI, and therefore, STI testing should be considered in all symptomatic adolescent ED female patients. Future studies should focus on understanding barriers to STI testing and designing interventions to increase testing within an adolescent ED population.