Mental and Behavioral Health Among Vulnerable Adolescents and Young Adults in the United States - Modified Project Summary/Abstract Section Most sexual minority adolescents and young adults (AYA) are attracted to more than one sex or to people regardless of sex (e.g., bisexual). Limited prior research suggests that bisexual AYA are at greater risk than their lesbian, gay, and heterosexual peers for adverse mental and behavioral health (MBH) outcomes, in part due to anti-bisexual stigma, a unique form of stigma enacted by both heterosexual and lesbian/gay individuals and communities. We will use a community-engaged sequential mixed methods study design to: (1) Examine how sexual orientation-based stigma is experienced by bisexual AYA. We will collect data via asynchronous online focus groups from AYA who are attracted to more than one sex. (2) Investigate the longitudinal impact of sexual orientation-based stigma on MBH outcomes considering developmental and sociodemographic factors. We will collect longitudinal survey data from bisexual AYA and will recruit a parent/caregiver among a subsample of youth to examine family-related factors that impact bisexual AYA MBH. (3) Determine key protective factors that reduce the adverse impact of sexual orientation-based stigma on MBH outcomes over time. We will utilize Aim 1 focus group data and Aim 2 survey data to examine the role of protective factors in mitigating the adverse impacts of stigma on MBH outcomes among bisexual AYA. The expected outcomes will be a novel understanding of how sexual orientation-based stigma is experienced by AYA, its longitudinal impact on AYA’s MBH, and how protective factors may prevent adverse MBH outcomes in this population. This research is a critical step toward preventing health disparities and improving the health and transition into adulthood for vulnerable AYA.