PROJECT SUMMARY Hospitalmergers and acquisitions (M&As) increase concentration in hospital markets.In 2016, 90% of Metropolitan Statistical Areas were considered “highly concentrated.”5 Much of the literature on hospital M&As focuses on the effects on private prices and hospital cost reductions.1,6–14 However, despite growing concern from federal and state regulatory agencies,2,3,15–17 little is known about how M&As affect access to care, especially for low-income Medicaid and uninsured patients, who most often experience access challenges.18–21 In an effort to improve efficiency and reduce costs, consolidating hospitals may eliminate a service line (i.e., whether a service is offered) at one of the M&A hospitals if it is provided at the other, particularly if the service line is unprofitable.22,23 This could reduce local capacity to provide clinically important services and, in turn, patients' geographic access. In addition, hospital MAs often result in increased private reimbursement.1,24,25 The economic theory of price discrimination suggests that hospitals seeking to maximize profits may reduce provision of care to Medicaid and uninsured patients. However, the theory of cross-subsidization suggests that public and not-for-profit hospitals would instead use increased profits from private patients to further subsidize care of Medicaid and uninsured patients and unprofitable services in line with their mission.26 These theories suggest the need to examine M&A-related access changes on both an extensive margin (i.e., whether a service is provided) and intensive margin (i.e., quantity of services provided).This study will examine M&A- related changes in hospital service provision and access using national data on hospital characteristics from the American Hospital Association and Centers for Medicare & Medicaid Services along with discharge-level data from the Healthcare Cost and Utilization Project State Inpatient Databases for select states. First, I will determine whether hospitals reduce provision of select inpatient and outpatient service lines after a M&A, focusing on service lines that are generally unprofitable due to high utilization by Medicaid and uninsured patients, such as substance use, psychiatric, obstetric, and trauma services. Next, I will estimate the extent to which Medicaid and uninsured inpatient admissions change following a M&A, focusing on elective admissions. While regulations ensure access to emergency care, hospitals are not obligated to provide Medicaid and uninsured patients access to elective inpatient services, such as knee replacements and bariatric surgery. Thus, changes in elective admissions may best reflect hospitals' efforts to limit or expand care. Building on these results, I will look at whether M&As result in shifts of Medicaid and uninsured patients to neighboring hospitals and reduce overall access at the regional scale for these patients. This study addresses an important knowledge gap by examining the ...