Project Abstract/Summary The more than 1.1 million US firefighters constitute a large portion of the first responders who respond to domestic crises. The health, safety, and readiness of firefighters is pertinent for not only for themselves but also their fellow firefighters and the public they serve. Research on the health and safety of firefighters has increased over the years, including the health and safety of women and racial minorities, both significant minority groups in the fire service. Unfortunately, the health status of US firefighters who identify as a sexual or gender minority (SGM) is completely unknown. A study from the UK examined fire service culture and its impact on sexual minority firefighters and found a non-inclusive environment. Lawsuits making national news in the US depicts potentially significant discrimination and harassment of SGM firefighters. The stigmatization SGM firefighters face in the work environment may lead to chronic stress and mental health disorders. SGM individuals experience disparities in mental health, and firefighters experience high rates of mental health disorders and substance use. Further, SGM individuals have higher rates of suicide attempts, and recently more firefighters died from suicide than in the line of duty. However, it is unknown if the fire service environment exacerbates mental health disparities for SGM firefighters. The proposed mixed methods study will be conducted in three phases including: 1) interviews with SGM and non-SGM firefighters, leaders in the fire service, and representatives from national fire service organizations; 2) content analysis of SGM news organizations and firefighter specific content including trade journals, websites, social media, and organizational literature; 3) national surveys of both SGM and non-SGM firefighters to explore health determinants of mental health disparities. This innovative study is an important initial step to understand health determinants of mental health disorders in order to reduce disparities, to improve recruitment and retention of sexual and gender minority firefighters, and will be the first to document SGM health disparities and their intersections with race and gender in the fire service. The strengths of this R21 developmental application include strong support from national fire service organizations and a team with a long history of success working with this occupational population and with SGM individuals.