 This is a twelve-week randomized, double blind, placebo controlled trial of topiramate for 40 patients with PTSD. Diagnoses are made by Structured Clinical Interview for DSM-IV SCID. Symptom severity is measured with the Clinician Administered PTSD Scale CAPS, Davidson Trauma Scale DTS, Treatment Outcome PTSD TOP-8, Clinical Global Impressions CGI, and the Hamilton Rating Scales for Depression and Anxiety HAM-D and HAM-A. The Clinician Administered PTSD Scale CAPS is the primary outcome measure. Subjects will read and sign informed consent prior to beginning any study procedures. Subjects who meet inclusionexclusion criteria will be randomized 1:1 Topiramate : placebo to study medication. Study medication is initiated at one tablet daily 25mg topiramate or placebo and medication will be increased by 25-100mg 1 tablet increments every 7 days, as tolerated, until a maximum benefit is achieved, not to exceed 400mgday. Patients will be seen every week up until Visit 4, and then will be seen every two weeks for the remainder of the study for assessment of efficacy and side effects. 