Objectives: Despite the U.S. Department of Veterans Affairs' (VA) expansion of mental health services to treat VA service users with post-traumatic stress disorder (PTSD), many with PTSD do not engage in treatment. Numerous studies suggest that beliefs about treatment and social network factors, such as encouragement to seek treatment by others, affect engagement; however, prospective studies examining these factors are largely absent in this population. This study sought to understand social and attitudinal factors influencing treatment initiation, which may help to inform outreach interventions for VA service users with PTSD. Methods: A prospective, national cohort study of mental health care use among veterans recently diagnosed as having PTSD (N=7,645) was undertaken. Data from self-administered surveys and administrative databases were analyzed to assess contributions of treatment-related beliefs and social network encouragement to subsequent mental health care use, after facility, demographic, need, and access factors were controlled. Results: After the analysis controlled for treatment need and accessibility, the odds of initiatingmental health care were greater for veterans who believed that they needed help for PTSD or other emotional problems and those who were encouraged to seek help by friends and family. Beliefs about the effectiveness of PTSD treatments were associated with the type of treatment received. Negative illness identity was not a barrier to treatment initiation. Conclusions: VA service users' social networks, veterans' perceptions of their need for mental health care, and their beliefs about PTSD treatment effectiveness may be fruitful targets for future treatment engagement interventions.