OBJECTIVES: Medicaid expansion under the Patient Protection and Affordable Care Act (ACA) will add new enrollees to Medicaid programs in states that elect to expand eligibility. The objective of this study is to provide projections of inpatient hospital and emergency department (ED) use after ACA Medicaid expansion. METHODS: Hospital Inpatient and ED records were extracted from Healthcare Cost and Utilization Project State Inpatient Databases for the years 2007–2011and State Emergency Department Databases for the years 2007–2010. The enrollment estimates were based on the Centers for Medicare & Medicaid Services Medicaid statistics and information from the American Community Surveys for 2007–2011. Inpatient discharge records were aggregated by the state of the patient’s residence, year, and major service lines including Medicine, Surgery, Maternity & Newborn, Injuries and Mental Health.  Data were restricted to adults aged 19–64 years, because this age group is likely to contribute the vast majority of new Medicaid enrollees. Regression models estimated utilization measures from predictor variables. Hospital utilization metrics were total discharges, preventable admissions, and emergency department visits. Discharge and ED visit rates were estimated using the state- and year-specific Medicaid enrollment estimates. Data for Medicaid patients were aggregated by state, year, and type of service. RESULTS: Our models project that change in population composition alone results in a 22% increase in inpatient discharges and a 30% increase in ED visits, while use rates fall 6% and 0%, respectively.  With the additional capacity, reimbursement, and innovation policy effects in place, inpatient discharges increase by 7% and ED visits by only 1%, while use rates fall 18% and 22%.    CONCLUSIONS: Medicaid expansion will increase inpatient and ED volumes, but utilization rates will be below current levels.  States can limit increases through provider capacity, Medicaid managed care, and increasing physician acceptance of Medicaid patients.