This is a multi-center, two-phase study to evaluate the efficacy and safety of risperidone augmentation to sertraline treatment of Post-Traumatic Stress Disorder (PTSD). In the first phase, all patients will be assigned to take open label sertraline for 8 weeks (up to 200 mg/d). Those who have not achieved a significant decrease in their PTSD symptoms by week 8 will be entered into the second phase. In the second phase, patients will continue with the sertraline, but will then be randomly given either risperidone (up to 3 mg/d) or matching placebo in double-blind fashion.