OBJECTIVE To understand what predicts good outcome in psychiatric treatments, thus creating a pathway to improving efficacy. METHODS Our study investigated relations between predictor variables and outcome (on the Clinician Administered Posttraumatic Stress Disorder [PTSD] Scale [CAPS]) at posttreatment for 77 treatment completers in a randomized controlled trial of exposure therapy and (or) cognitive restructuring, compared with relaxation, for chronic PTSD in adults. RESULTS More social support on the Significant Others Scale significantly predicted better outcome on the CAPS, even after controlling for the effects of treatment group and of pretreatment severity. Importantly, social support was only a significant predictor of outcome for participants receiving cognitive restructuring and (or) exposure therapy and not for participants in the relaxation condition. CONCLUSIONS Better social support is associated with significantly greater gain following cognitive restructuring and (or) exposure therapy for PTSD. Future interventions should consider augmenting social support as an adjunct to treatment.