Project Summary Gay, bisexual, and other sexual minority men (SMM) are the group most impacted by the domestic HIV epidemic. Critically, these rates are highest among young SMM (YSMM), with Black, Latino, and White YSMM under age 35 being the three groups with the highest rates of new HIV diagnoses in 2016. Although more is known about sexual minority stigma (i.e., internalized homonegativity), little quantitative work has focused on intersectional experiences of stigma for diverse YSMM. Compared to White YSMM, intersectionality theory suggests that increased HIV incidence among Black and Latino YSMM may due in part to the distinct effects of stigma that occurs at the intersection of their multiple stigmatized identities. Researchers have consistently found that exposure to sexual minority stigma at least partially explains the disproportionate experience of adverse mental and physical health outcomes for SMM compared to the general population, though the literature on intersectional stigma and HIV prevention for YSMM is less well developed. Experiences of stigma occur regularly and are shaped by the context in which they occur, and event-level methodologies may be an ideal way to collect such data. Although a few event-level studies of stigma have been published, few if any have used rigorously developed, community-informed, and psychometrically validated items. To address these limitations of the literature to date, the goal of this research is to conduct a three-phase study to generate, refine, and optimize event-level measures of stigma for Black, Latino, and White YSMM aged 16-29 that focus on the intersection of race, ethnicity, sexual minority identity, and gender performance (i.e., masculinity). The first aim of the study is to generate and refine an item pool using the available literature and interviews with expert researchers and community members (Phase 1) followed by an iterative process of cognitive interviewing and pilot testing of the items with YSMM themselves (Phase 2). Having generated an item pool, the second aim of the study is to use the items to conduct a 21-day daily diary study with 150 YSMM (Phase 3) to examine the psychometric properties of the items, reduce the item pool, and test the predictive validity of the items for three HIV prevention outcomes—HIV testing, STI testing, and rectal STI infection. To enroll YSMM for Phases 2 and 3, we will leverage an existing cohort to rapidly recruit participants; in Phase 3, we will also gather data collected during the cohort study both prior to and following the 21-day diary study. The proposed study will address a notable gap in the literature by systematically developing and optimizing items to capture event-level experiences of intersectional stigma for diverse YSMM across the U.S. Overall, the final item pool developed will help to further research on intersectional stigma and health disparities; generating items with greater cultural relevance and lower measurement error ...