Importance: Trauma is the leading cause of death and disability among young adults, who are also among the most likely to be uninsured. Efforts to increase insurance coverage, including passage of the Patient Protection and Affordable Care Act (ACA), were intended to improve access to care and promote improvements in outcomes. However, despite reported gains in coverage, the ACA's success in promoting use of high-quality care and enacting changes in clinical end points remains unclear. Objectives: To assess for observed changes in insurance coverage and rehabilitation use among young adult trauma patients associated with the ACA, including the Dependent Coverage Provision (DCP) and Medicaid expansion/open enrollment, and to consider possible insurance and rehabilitation differences between DCP-eligible vs - ineligible patients and among stratified demographic and community subgroups. Design, Setting, and Participants: A longitudinal assessment of DCP implementation and Medicaid expansion/open enrollment using risk-adjusted before-and-after, difference-in-difference, and interrupted time-series analyses was conducted. Eleven years (January 1, 2005, to September 31, 2015) of Maryland Health Services Cost Review Commission data, representing complete patient records from all payers within the state, were used to identify all hospitalized young adult (aged 18-34 years) trauma patients in Maryland during the study period. Results: Of the 69 507 hospitalized patients included, 50 548 (72.7%) were male, and the mean (SD) age was 25 (5) years. Before implementation of the DCP, 1 of 4 patients was uninsured. After ACA implementation, the number fell to less than 1 of 10, with similar patterns emerging in emergency department and outpatient settings. The change was primarily driven by Medicaid expansion/open enrollment, which corresponded to a 20.1 percentage-point increase in Medicaid (95% CI, 18.9-21.3) and an 18.2 percentage-point decrease in uninsured (95% CI, -19.3 to -17.2). No changes were detected among privately insured patients. Rehabilitation use increased by 5.4 percentage points (95% CI, 4.5-6.2)-a 60% relative increase from a baseline of 9%. Mortality (-0.5; 95% CI, -0.9 to -0.1) and failure-to-rescue rates (-4.5; 95% CI, -7.4 to -1.6) also significantly declined. Stratified changes point to significant differences in the percentage of uninsured patients and rehabilitation access across the board, mitigating or even eradicating disparities in certain cases. Conclusions and Relevance: For patients who are injured, young, and uninsured, Medicaid expansion/open enrollment in Maryland changed insurance coverage and altered patient outcomes in ways that the DCP alone was never intended to do. Implementation of Medicaid expansion/open enrollment transformed the landscape of trauma coverage, directly affecting the health of one of the country's most vulnerable at-risk groups.