The purpose of this study is to (1) compare the response of civilians with Posttraumatic Stress Disorder(PTSD) currently receiving sertraline without an optimal response to risperidone augmentation vs. placebo, and (2) to evaluate the tolerability of risperidone augmentation, and (3) to identifiy predictors of response to risperidone augmentation. the hypothesis is that risperidone augmentation of sertraline treatment of PTSD is safe and effective.