Breast cancer (BC) survivors frequently use online support groups (OSGs) to help cope with their cancer, but evidence on the efficacy of OSGs for reducing psychological distress is weak. The proposed study assesses the potential efficacy (estimate effect sizes), acceptability, feasibility, and mediators of an innovative "prosocial" OSG (PRO-OSG) for distressed BC survivors. PRO-OSG provides structured helping opportunities (blogs, outreach) and coaching on how to recognize needs and give support to others in a BC OSG. The project is based on the "helper therapy principle," which emphasizes the psychological benefits of giving (vs. receiving) help. This is the first study to evaluate a prosocial intervention in the context of an OSG. In Aim 1, we propose using a randomized trial to assess the efficacy and effect sizes of PRO-OSG relative to a standard facilitated OSG (S-OSG). We will collect data from distressed BC survivors 1 month pre- and post-intervention using valid and reliable measures appropriate for the population. Intention-to-treat analyses will be used to compare the effects of the two interventions on levels of depression/anxiety symptoms (primary outcomes) and sense of purpose (secondary outcome) (n = 180;90 per group). We hypothesize that relative to S-OSG, PRO-OSG will have a lower level of symptoms and a higher level of sense of purpose post-intervention. Accuracy in parameter estimation techniques (AIPE) will be used to assess effect sizes on outcomes. In Aim 2, we will explore: (a) if the benefits of PRO-OSG on psychological symptoms are mediated by greater increases in positive affect, self-esteem, and sense of belonging among participants in the PRO-OSG versus S-OSG group;(b) if the benefit of PRO-OSG on sense of purpose is mediated by a greater increase in sense of belonging in the PRO-OSG versus S-OSG;and (c) if sense of belonging indirectly improves psychological symptoms by increasing sense of purpose. AIPE will be used to assess effect sizes on mediators. In Aim 3, we will examine acceptability (e.g., retention, satisfaction) and feasibility (e.g., amount of helping) of the PRO- OSG. The significant outcome of this project will be an innovative, theory-based OSG intervention model that maximizes reductions in psychological distress and increases sense of purpose among distressed BC survivors. The intervention is sustainable, because it can be implemented by organizations such as the Cancer Support Community, a partner on this project and the largest provider of free psychosocial support services for cancer survivors in North America. The intervention also can be adopted by other organizations, such as cancer centers, which to date have mostly offered face-to-face support groups with limited patient reach. Future directions include developing a larger-scale trial to further evaluate PRO- OSG;evaluating PRO-OSG in other cancer populations;and developing and assessing the efficacy of PRO-OSG training protocols that will enable peer leaders (vs. helping professionals) to facilitate OSGs. PUBLIC HEALTH RELEVANCE: Depression and anxiety symptoms and disorder are highly prevalent among BC survivors and often undertreated. Depression is more prevalent among BC survivors than many other cancers populations, but existing interventions are not widely acceptable, accessible, or highly effective. The proposed study will develop and evaluate an innovative "prosocial" online support group that provides structured helping opportunities (blogs, outreach) and coaching on how to recognize needs and give support to others in an online support group for distressed BC survivors.