AbstractObjectiveTo examine changes in hospitalization trends and healthcare utilization among Veterans following Maintaining Internal Systems and Strengthening Integrated Outside Networks (MISSION) Act implementation.Data Sources and Study SettingVA Corporate Data Warehouse and Centers for Medicare and Medicaid Services datasets.Study DesignRetrospective cohort study to compare 7‐ and 30‐day rates for unplanned readmission and emergency department visits following index hospital stays based on payor type (VHA facility stay, VA‐funded stay in community facility [CC], or Medicare‐funded community stay [CMS]). Segmented regression models were used to compare payors and estimate changes in outcome levels and slopes following MISSION Act implementation.Data Collection/Extraction MethodsVeterans with active VA primary care utilization and ≥1 acute hospitalization between January 1, 2016 and December 31, 2021.Principal FindingsMonthly index stays increased for all payors until MISSION Act implementation, when VHA and CMS admissions declined while CC admissions accelerated and overtook VHA admissions. In December 2021, CC admissions accounted for 54% of index admissions, up from 25% in January 2016. From adjusted models, just prior to implementation (May 2019), Veterans with CC admissions had 47% greater risk of 7‐day readmission (risk ratio [RR]: 1.47, 95% confidence interval [CI]: 1.43, 1.51) and 20% greater risk of 30‐day readmission (RR: 1.20, 95% CI: 1.19, 1.22) compared with those with VHA admissions; both effects persisted post‐implementation. Pre‐implementation CC admissions were also associated with higher 7‐ and 30‐day ED visits, but both risks were substantially lower by study termination (RR: 0.90, 95% CI: 0.88, 0.91) and (RR: 0.89, 95% CI: 0.87, 0.90), respectively.ConclusionsMISSION Act implementation was associated with substantial shifts in treatment site and federal payor for Veteran hospitalizations. Post‐implementation readmission risk was estimated to be higher for those with CC and CMS index admissions, while post‐implementation risk of ED utilization following CC admissions was estimated to be lower compared with VHA index admissions. Reasons for this divergence require further investigation.