Project Summary There is growing policy activity among states and counties to use non-federal funds to offer health care coverage to low-income, uninsured undocumented immigrants. However, there is only limited evidence on undocumented immigrants’ health care utilization in the context of expanded access to care to support these policies and guide their implementation. This R21 project will investigate one long-standing local program to build the evidence base on how expanded access to care affects undocumented immigrants’ patient outcomes and health care utilization. We will focus on patients enrolled in MyHealth LA (MHLA), the oldest and largest local health plan for undocumented immigrants, located in Los Angeles County. MHLA provides a primary care home at Federally Qualified Health Centers (FQHCs) and specialty, inpatient, and ED care through the county health system for ~130,000 patients annually. This project will determine the impact of programs that expand health care access to uninsured, low-income undocumented immigrants and understand how these patients utilize care over time. Aim 1 of our project will use a unique data source of merged patient records, Medi-Cal claims, and enrollment records from hospital encounters to determine whether MHLA patients have better (1) ED and (2) inpatient outcomes than a matched sample of undocumented patients who are not enrolled in MHLA (ED encounters>140,000 , inpatient encounters>46,000) between 2016-2022. This aim will provide evidence on the impact of expanded access to care on patient outcomes by comparing enrolled and non- enrolled patients. Aim 2 will look within the entire enrolled MHLA population to understand utilization over time. We will merge MHLA enrollment records with Medi-Cal claims data from 2015-2022 to construct individual- level panel data (n>90,000) to examine (1) ED, (2) outpatient specialty, (3) primary care, (4) hospitalized inpatient, (5) preventive care, and (6) prescription utilization patterns 24 months after enrollment. We will identify differences in utilization by important demographic characteristics (e.g race/ethnicity, age, language, chronic conditions). This information can be used to distinguish subgroups with high utilization, set program standards, and anticipate utilization patterns over time. We will also examine whether clinic characteristics and external events (ie, COVID-19 pandemic) correlate with utilization patterns. The result of this study can be used to help support the development of local and state-level policies that expand access to care for undocumented residents as well as help policy makers, clinicians, and researchers anticipate outcomes after these policies are implemented. We will work with a Steering Committee of FQHC leadership, MHLA leadership, and community advocates to interpret and disseminate our findings.