OBJECTIVES: For SCD patients, inadequate care during pediatric to adult transition may result in increased emergency department (ED) utilization. Emergency department reliance (EDR: total ED visits/total ambulatory [outpatient + ED] visits) identifies the proportion of ED visits in relation to all ambulatory visits. This study aimed at investigating age-related patterns of EDR and associated healthcare costs in SCD patients.  METHODS: State Medicaid data from Florida, New Jersey, Missouri, Iowa, and Kansas were analyzed.  Patients with ≥2 SCD diagnoses (ICD-9 282.6x) and ≥1 blood transfusion were included. Quarterly rates of EDR and SCD complication-related ED visits as well as healthcare costs were evaluated. Based on published thresholds, high EDR was defined as >0.33. Regression analyses were used to assess risk factors for high EDR and calculate adjusted costs difference between patients with high versus low EDR. RESULTS: A total of 3208 patients were identified; mean (SD) observation period was 6.5 (3.2) years. Mean ED visits/quarter increased from 0.76 to 2.23 between age 15 and 23, reaching a peak of 2.9 at age 36. The most common SCD complication-related ED visits were pain, infection, and pneumonia. EDR rose from 0.15 to 0.29 between age 15 and 23, and remained high thereafter. Patients were more likely to have high EDR during the post-transition period (≥18 years old, odds ratio [OR]: 2.38, p<0.001) and when experiencing an SCD complication (OR: 4.18, p<0.001). Patients with high EDR incurred higher inpatient and ED costs, resulting in higher total costs (high vs. low EDR, adjusted costs difference, OP: -$285; IP: $3,485; ED: $120; Rx: -$91; total: $3,086, p<0.001 for all). CONCLUSIONS: Compared to children, SCD patients transitioning to adulthood relied more on ED for their care and those with high EDR incurred higher health care costs, highlighting the need to improve access to care for transitioning and adult SCD patients.