OBJECTIVES: Adolescent females are disproportionately affected by sexually transmitted infections (STIs). Although Trichomonas vaginalis (TV) has been declared the most common nonviral STI, TV testing is not routinely conducted in the emergency department (ED), and when it is performed, insensitive testing methods are often used. Therefore, this study sought to determine the prevalence of TV and factors associated with TV infection among symptomatic adolescent females presenting to a pediatric ED. METHODS: This was a prospective prevalence study of female patients ages 14 to 19 years presenting to a pediatric ED with lower abdominal pain and/or genitourinary (GU) complaints. Patients were tested for TV, Neisseria gonorrhoeae (GC), and Chlamydia trachomatis (CT). RESULTS: Of the 276 patients who met inclusion criteria, 203 underwent TV testing; prevalence was 9.9% (95% confidence interval [CI] = 5.7% to 14.0%). Assuming all eligible patients who did not have TV testing were not infected with TV, sensitivity analysis revealed a minimum TV prevalence of 7.2% (95% CI = 4.2% to 10.3%). The overall prevalence of any STI was 22.5% (95% CI = 17.5% to 27.4%), with CT being the most prevalent (19.7%; 95% CI = 14.5 to 24.9). Aside from vaginal discharge (odds ratio [OR] = 3.7; 95% CI = 1.1 to 11.3), there were no other factors significantly associated with TV infection. CONCLUSIONS: A substantial proportion of adolescent females presenting to a pediatric ED with lower abdominal or GU symptoms had TV infection. TV testing should be considered as part of the evaluation of adolescent females presenting to the ED with lower abdominal, urinary, or vaginal complaints.