Abstract:Purpose: Due to widespread stigma in the military, many military servicemembers with mental illness struggle with discrimination, self-stigma and decisions whether to disclose their condition. This study had the aim to evaluate the efficacy of Honest, Open, Proud (HOP), a four-session peer-led group program adapted to support military personnel with disclosure decisions and to reduce stigma’s impact, in the German military. Methods: Ninety-nine active servicemembers with mental illness were randomized to HOP and treatment as usual (TAU) or to TAU alone. The two primary endpoints were stigma stress three weeks after baseline (T1/after HOP for HOP participants) and psychological quality of life six weeks after baseline (T2/after the HOP booster session). This randomized-controlled trial was registered before recruitment onset at ClinicalTrials.gov (NCT03218748). Results: Compared to the control group, stigma stress decreased significantly among HOP participants at T1 (d=0.64), while there was no significant effect of HOP on psychological quality of life at T2. HOP had significant positive effects on secondary outcomes at T2: overall quality of life, self-stigma, depressive symptoms, empowerment, well-being, attitudes to disclosure and to help-seeking, and secrecy. Reductions in stigma stress, secrecy, and marginally in overall quality of life (p=0.055), remained significant twelve weeks after baseline (T3). Conclusion: This trial provides initial evidence that HOP for active military servicemembers is feasible and effective in terms of stigma stress, disclosure decisions, depressive symptoms, quality of life and well-being. HOP may be a valuable addition to mental health services and peer support in the military.