This study examines the effect of the Affordable Care Act's Medicaid expansion on hospital financial outcomes. A key innovation relative to prior studies is that we explicitly account for heterogeneity across states in the timing and extent of the expansion as well as across hospital types. We find that Medicaid expansion led to a decrease in uncompensated care expenditures and an increase in average operating margins. The effects were larger in states where the Medicaid expansion led to a greater increase in program eligibility. Operating margins improved most for public hospitals and facilities located in rural areas. (JEL I11, I13, I18).