Purpose: Adolescents are commonly diagnosed with sexually transmitted infections (STI) in the emergency department (ED). Untreated, STIs can lead to morbidity such as pelvic inflammatory disease (PID) and infertility. Although many patients are presumptively treated in the ED with single dose antibiotics, many are prescribed outpatient treatment. Rates of prescription filling after discharge are unknown. This study sought to determine the frequency of filled prescriptions for adolescents prescribed antibiotics for outpatient management of STIs in an urban pediatric ED and identify factors associated with prescription filling. Methods: We performed a retrospective medical record review of all 2016 ED visits made by adolescents aged 13-19 years who were prescribed antibiotics for the treatment of Chlamydia trachomatis (CT) or Neisseria gonorrhea (GC) cervicitis/urethritis or PID in an urban pediatric ED. We extracted data on outpatient prescription filling, patient demographics, and visit characteristics. We calculated the proportion of outpatient prescriptions filled, and performed multivariable logistic regression to identify patient- and visit-level factors associated with prescription filling. Results: During the study period, 337 patients were diagnosed with an STI, of whom 127 (37.7%) were prescribed outpatient antibiotics for CT/GC (n=42) or PID (n=85) treatment. The mean age was 17.1 (+/- SD 1.4) years, and the majority was female (89.8%), of non-Hispanic black race/ethnicity (78.7%), and publicly insured (85.0%). 18.1% of patients were admitted for inpatient management of PID. Of the 127 patients prescribed outpatient antibiotics, the overall fill rate was 46.5% (95% CI 37.7, 55.3) [CT/GC (43.3%; 95% CI 31.2, 55.4) and PID (50.5%; 95% CI 39.8, 61.4)]. After adjusting for age, race/ethnicity, and insurance status, prescription fill rates were significantly higher among admitted patients (aOR 3.3; 95% CI 1.2, 8.7) compared to patients discharged from the ED. Conclusion: Overall, adolescents evaluated in an urban pediatric ED had low rates of filling outpatient prescriptions for treatment of STIs, placing them at risk for increased morbidity and complications from untreated STIs, as well as further transmission of infection. Hospital admission was associated with higher prescription filling rates. Future studies should focus on understanding barriers to prescription filling to inform interventions for improvement in treatment adherence.