OBJECTIVES: In 2014, many states will initiate Medicaid expansion under the Affordable Care Act; some states will not. Medicaid expansion is expected to increase hospital utilization as previously uninsured adults become covered under Medicaid. Our objective is to define benchmarks for the rates of hospital inpatient and emergency department (ED) use following Medicaid expansion. METHODS: We obtained hospital use data from the Healthcare Cost and Utilization Project 2010 State Inpatient Databases and State Emergency Department Databases. We obtained state-level data on Medicaid program characteristics and population demographics and health status from Centers for Medicare & Medicaid Services Medicaid statistics, American Community Survey, and Behavioral Risk Factor Surveillance Survey. We examined whether hospital utilization rates differed based on states’ stance on Medicaid expansion. We standardized the hospital use metrics by computing separate index values for the Medicaid and uninsured population metrics relative to the national mean so that all measures had similar scale. We then examined which state-level Medicaid program, demographic, and health status characteristics were related to the states’ expansion stance. RESULTS: We found that several state health system infrastructure characteristics were strongly related to both expansion likelihood and hospital utilization. In particular, in states highly likely to adopt Medicaid expansion in 2014, we observed higher levels of Medicaid managed care organization penetration, a lower primary care physician supply challenge, a lower level of primary care case management, and a smaller expansion population size relative to the current Medicaid population. We also found that in those states that are currently committed to Medicaid expansion had substantially lower hospital inpatient and ED use among Medicaid-covered patients compared to states that have not committed to expand. CONCLUSIONS: Our results revealed a lower impact of Medicaid expansion on hospital utilization among states that have elected to expand than among states currently unlikely to expand.