PROJECT SUMMARY/ABSTRACT In response to PAR-18-747, Addressing the Challenges of the Opioid Epidemic in Minority Health and Health Disparities Research in the U.S. (R01), we propose longitudinal analyses of gender difference in entry, access, and engagement in medication-assisted treatment (MAT) for opioid use disorder (OUD). Our study team includes leading experts in gender disparities (Drs. Marsh and Grella), MAT access (Drs. Guerrero and Hortensia Amaro) and advanced longitudinal approaches (Drs. Kong and Hedeker). We will draw from unique multiyear, program- and client-level data from before (2011-2013) and after (2015-2017) Medicaid expansion to detect disparities and examine the role of risks to women, program capacity, and comprehensive services in eliminating disparities overtime. In Aim 1, we will detect gender differences in trends in treatment entry (clients with OUD), access (wait time), and engagement (retention and dropout risk) in Los Angeles County from 2011- 2017. This will provide the first-known evidence of gender-based trends in treatment entry, access, and engagement in MAT in one the largest publicly funded treatment systems in the United States. In Aim 2, we will seek to understand differences between men and women and among women with OUDs by risk profiles and program factors across key outcomes of treatment access and engagement. This will provide the first-known evidence of differential pathways for women and men to access and engage in MAT programs. In Aim 3, we will examine the effect of Medicaid expansion on women with OUDs, controlling for client risk profiles and program capacity, across outcomes of treatment entry, access, and engagement. This work will document the impact of Medicaid expansion on women's MAT entry, access and engagement. Our findings will inform health care policies and practices and advance systems science and practice guidelines to enhance access to and engagement in care for women with OUD, one of the most vulnerable populations affected by the current opioid epidemic.