Objective Ongoing health care reforms within the US Military Health System (MHS) are expected to shift >1.9 million MHS beneficiaries from military treatment facilities (MTFs) into local civilian hospitals over the next 1-2 years. The objective of this study was to examine how such health care reforms are likely to affect the quality of MHS care. Data sources Adult MHS beneficiaries, aged 18-64 years, treated in MTFs (under a program known as Direct Care) were compared against (1) MHS beneficiaries treated in locally available civilian hospitals (under a program known as Purchased Care) and (2) similarly-aged adult civilian patients across the United States. MHS beneficiaries in Direct and Purchased Care were identified from fiscal-year 2016-2018 MHS inpatient claims. National inpatients were identified in the 2017 Nationwide Readmissions Database. Study design Retrospective cohort. Data collection Differences in quality were compared using two sets of quality metrics endorsed by the US Agency for Healthcare Research and Quality (AHRQ): Inpatient Quality Indicators, 19 quality metrics that look at differences in in-hospital mortality, and Patient Safety Indicators, 18 quality metrics that look at differences in in-hospital morbidity and adverse events. Among MHS beneficiaries (Direct and Purchased Care), we further simulated what changes in quality indicators might look like under various proposed scenarios of reduced access to Direct Care. Principal findings A total of 502,252 MHS admissions from 37 MTFs and surrounding civilian hospitals were included (326,076 Direct Care, 179,176 Purchased Care). Nationwide, 9.34 million adult admissions from 2453 hospitals were included. On average, MHS beneficiaries treated in MTFs experienced better inpatient quality and improved patient safety compared with MHS beneficiaries treated in locally available civilian hospitals (e.g., summary observed-to-expected ratio for medical mortality: 0.98 vs. 1.03, p < 0.001) and adult patients across the United States (0.98 vs. 1.02, p < 0.001). Simulations of proposed changes resulted in consistently worse outcomes for MHS patients, whether reducing MTF access by 10%, 20%, or 50% nationwide; limiting MTF access to active-duty beneficiaries; or closing MTFs with the worst performance on patient safety (p < 0.001 for overall quality indicators for each). Conclusions Reducing access to MTFs could result in significant harm to MHS patients. The results underscore the importance of health-policy planning based on evidence-based evaluation and the need to consider the consequential downstream effects caused by changes in access to care.