AIM: To validate the first third-person-rated measure assessing combat-related peritraumatic stress symptoms and evaluate its psychometric properties and war-zone applicability.METHODS: The valid assessment of peritraumatic symptoms in the theater of military operations represents a significant challenge in combat-related, mental health research, which mainly relies on retrospective, subjective self-report ratings. This longitudinal observational study used data from actively deployed troops to correlate third-person observer ratings of deployment peritraumatic behaviors [Peritraumatic Behavior Questionnaire - Observer Rated (PBQ-OR)] collected on a bi-monthly basis with post-deployment (1-wk follow-up) ratings of the previously validated PBQ self-rate version (PBQ-SR), and (3-mo follow-up) clinician assessed and self-report posttraumatic stress disorder (PTSD) symptoms (Clinician Administered PTSD Scale, PTSD Checklist). Cronbach's alpha (α) and correlation coefficients were calculated to assess internal reliability and concurrent validity respectively.RESULTS: Eight hundred and sixty male Marines were included in this study after signing informed consents at pre-deployment (mean age 23.2 ± 2.6 years). Although our findings were limited by an overall sparse return rate of PBQ-OR ratings, the main results indicate satisfactory psychometric properties with good internal consistency for the PBQ-OR (α = 0.88) and high convergent and concurrent validity with 1-wk post-deployment PBQ-SR ratings and 3-mo posttraumatic stress symptoms. Overall, later PBQ-OR report date was associated with higher correlation between PBQ-OR and post-deployment measures. Kappa analysis between PBQ-OR and PBQ-SR single items, showed best agreement in questions relating of mortal peril, desire for revenge, and experience of intense physical reactions. Logistic regression demonstrated satisfactory predictive validity of PBQ-OR total score with respect to PTSD caseness (OR = 1.0513; 95%CI: 1.011-1.093; P = 0.02).CONCLUSION: Since no comparable tools have been developed, PBQ-OR could be valuable as real-time screening tool for earlier detection of Service Members at risk.