Food insecurity and unstable housing are associated with obesity, diabetes, and cardiovascular disease. Medicaid programs and health systems are investing billions of dollars to address health-related social needs and improve health equity. Despite this investment, there are knowledge gaps about the effectiveness of health care-based interventions to address food, housing, or other social needs. This study will extend and expand on our current evaluation of a Massachusetts Medicaid (MassHealth) food and housing insecurity program, using a natural policy experiment to determine the program’s impact on health-related social needs, health, and health care use and spending. In 2017, MassHealth received a 1115 demonstration waiver that included funding for the Flexible Services program (Flex) to address enrollees’ food and housing insecurity. The Flex pilot was implemented between March 2020 and March 2023, and the LiveWell study (R01 DK124145, Thorndike) has been assessing Flex outcomes within a large accountable care organization (ACO). Although LiveWell data will not be complete until 2024, early implementation evaluation findings suggest that contextual and implementation factors, including the COVID-19 pandemic, may have hindered program implementation. Nevertheless, preliminary survey data and qualitative interviews with study participants indicate that many of those who received Flex services experienced improvements in food security, stress, diet, and health. A renewed 1115 waiver will extend Flex through 2027, providing a timely opportunity to assess Flex after the 3- year ramp-up pilot and beyond the acute phase of the pandemic. In the waiver extension, Flex nutrition services are limited to 6 months per enrollee (vs. up to 24 months in the pilot), allowing for standardized assessment of immediate and sustained program effects. The current project builds upon the research team’s experience in the LiveWell study to answer critical questions about the impact of Flex as it moves beyond the pilot phase in the 1115 waiver extension. First, we will leverage a target trial emulation framework using electronic health record and claims data from the Mass General Brigham ACO to examine the impact of Flex on acute health care utilization and medical spending among all ACO members and on medication adherence and health outcomes for adults with type 2 diabetes and hypertension (Aim 1). Second, we will prospectively enroll 500 Flex enrollees and 500 matched non-Flex ACO members to examine the longitudinal impact of Flex nutrition services on food security, diet, and psychological outcomes before, during, and after Flex (Aim 2). Finally, we will continue our rigorous mixed methods implementation evaluation using the RE-AIM/PRISM framework, expanding to examine maintenance of Flex as it becomes more established in the waiver extension (Aim 3). Our rigorous evaluation of this natural experiment will provide essential data to guide other states and health sys...