Background: PTSD is a significant problem for Veterans, most of whom are initially treated for posttraumatic stress disorder (PTSD) in Primary Care Mental Health Integration (PCMHI). Poor social support in this population is a risk factor for suicidal ideation, all-cause mortality, and worse treatment engagement and response. Preliminary research shows that approximately 60% of Veterans with PTSD present to specialty care intake appointments with poor social support. However, none of the existing PCMHI-based treatments for PTSD explicitly focus on social support generation or reengagement. Moreover, although the limited existing research suggests that the routine assessment of social support to guide treatment planning (also known as measurement-based care, or MBC) improves treatment outcomes in civilian populations, social support is not routinely monitored as part of evidence-based treatments for PTSD. Thus, this CDA-2 will first identify an appropriate instrument for routinely measuring social support in the context of clinical care to improve PTSD treatment, and, subsequently, evaluate a novel PCMHI-based PTSD treatment as part of a stepped care model for Veterans who report poor social support and are at great risk of PTSD treatment non-response. Significance/Impact: The proposed research addresses several HSR\&D research priorities, including evaluating the impact of social determinants of health, namely, social support, on the quality and outcomes of care; testing new models of mental health care to improve outcomes; and intervening with vulnerable Veterans with PTSD and suicide risk. By measuring and improving social support to enhance PTSD treatment, the proposed research plan explicitly addresses a key factor that impedes treatment outcomes for a sensitive population of Veterans. Innovation: Although poor social support is widely recognized as among the strongest predictors of PTSD development and maintenance, none of the existing PCMHI-based treatments for PTSD target or measure social support as a primary treatment focus. The proposed CDA-2 research is thus highly novel. No past or present HSR\&D-funded studies have tested methods for monitoring or improving social support in PCMHI-based treatment among Veterans with PTSD, despite its association with poor clinical outcomes. Specific Aims: 1) Select an instrument for MBC of social support in clinical settings for Veterans with PTSD. 2) Iteratively refine and conduct a one-arm pilot test a brief, PCMHI-based behavioral activation and social engagement intervention for PTSD based on key stakeholder (e.g., Veteran and PCMHI provider) feedback. 3) Conduct a two-arm pilot randomized clinical trial (RCT) comparing the intervention to usual care in PCMHI. Methodology: Aim 1 will use quantitative data gathered in an online survey from 210 Veterans to assess the degree to which four measures of social support (identified via systematic review and clinical utility ranking) are acceptable, reliable, and sensitive, and select the best measure for use in Aim 2's pilot. Aim 2 will involve the iterative refinement of the PTSD and social support intervention following, and followed by, qualitative interviews with key stakeholders. Aim 2 will obtain data on participant acceptability, provider fidelity, and social-support-related MBC. The pilot RCT comparing the social support and PTSD intervention to usual care in Aim 3 will evaluate the feasibility and acceptability of recruitment, randomization, intervention engagement, evaluation strategy, and outcome measurement of clinical outcomes (e.g., PTSD, depression symptoms, and social support), as well as mechanisms of change (e.g., disclosure) and multiple stepped-care outcomes (e.g., clinically-indicated evidence-based psychotherapy initiation/retention). Next Steps/Implementation: This work will support future trials establishing effectiveness and implementation potential of the intervention, as well as a model of MBC of social support. Future research will explore MBC and PCMHI-based interventions for poor social support in related conditions.