Purpose: Honest, Open, Proud (HOP; formerly “Coming Out Proud”/COP) is a peer-led group program to support people with mental illness in their disclosure decisions and in their coping with stigma. The aims of this study were to provide (i) a conceptual review of HOP, including versions for different target groups and issues related to outcome measurement and implementation; and (ii) a meta-analysis of program efficacy. Methods: Conceptual and empirical literature on disclosure and the HOP program was reviewed. Controlled trials of HOP/COP were searched in literature databases. A meta-analysis of HOP efficacy in terms of key outcomes was conducted. Results: HOP program adaptations for different target groups (e.g. parents of children with mental illness; veterans or active soldiers with mental illness) exist and await evaluation. Recruitment for trials and program implementation may be challenging. A meta-analysis of five HOP RCTs for adults or adolescents with mental illness or adult survivors of suicide attempts found significant positive effects on stigma stress (smd = − 0.50) as well as smaller, statistically non-significant effects on self-stigma (smd = − 0.17) and depression (smd = − 0.11) at the end of the HOP program. At 3- to 4-week follow-up, there was a modest, not statistically significant effect on stigma stress (smd = − 0.40, 95%-CI -0.83 to 0.04), while effects for self-stigma were small and significant (smd = − 0.24). Long-term effects of the HOP program are unknown. Conclusion: There is initial evidence that HOP effectively supports people with mental illness in their disclosure decisions and in their coping with stigma. Implementation issues, future developments and public health implications are discussed.