Inclusion Criteria:

• PTSD as defined by DSM-IV for at least 6 months supported by the Structured Clinical Interview for DSM-IV Axis I Disorders (SCID-I);
• Subjects must have a minimum past week CAPS score of > than 50 at Visit 2;
• Subjects must be in general good health as confirmed by medical history, and physical examination;
• Subjects must be off prohibited medications for washout periods as outlined under Concomitant Therapy section;
• Subjects must have negative urine drug screen (phencyclidine, cocaine, amphetamines, tetrahydrocannabinol, and opiates) at visit 1;
• Subjects must either be postmenopausal for at least 1 year,have had a hysterectomy or tubal ligation, be otherwise incapable of pregnancy or have practiced one of the following methods of contraception for at least one month prior to enrollment: hormonal contraceptives, spermicide and barrier, intrauterine device, spousal/partner sterility or be practicing abstinence and agree to continue abstinence or to use an acceptable method of contraception (as listed above) should sexual activity commence.
• Subjects must be able to take oral medication, adhere to medication regiments and be willing to return for regular visits;
• After full explanation of the study, subjects must demonstrate their willingness to participate by signing an informed consent form.

Exclusion Criteria:

• Subjects who have a DSM-IV diagnosis of substance dependence or abuse (with the exception of nicotine or caffeine dependence) within the past 3 months;
• Subjects with current or past history of primary major depressive disorder or primary major anxiety disorder (i.e. panic disorder, obsessive-compulsive disorder, social phobia) as defined by DSM-IV;
• Subjects with current or lifetime history of schizophrenia, bipolar disorder, or other psychotic disorder;
• Subjects with diagnosis of current organic mental disorder, factitious disorder or malingering
• Subjects with diagnosis of bulimia or anorexia nervosa;
• Subjects who are currently enrolled in cognitive-behavioral therapy;
• Subjects with current disability compensation or claim pending for persisting functional impairment related to PTSD;
• Subjects with prior non-response to topiramate for the treatment of PTSD following an adequate trial.
• Subjects who have previously been treated with topiramate and discontinued treatment due to an adverse event or subjects with a known hypersensitivity to topiramate;
• Subjects with clinically unstable disease: cardiovascular, renal, gastrointestinal, pulmonary, metabolic, endocrine or other systemic disease that could interfere with their participation in the study;
• Subjects with history of nephrolithiasis;
• Subjects with SGOT and/or SGPT levels greater than 2 times the upper limit of normal range at screening;
• Subjects taking antipsychotics within 3 months of the screening visit;
• Subjects with progressive or degenerative neurologic disorders (e.g. multiple sclerosis);
• Subjects with active liver disease;
• Subjects with estimated creatinine clearance < 60 mL/min;
• Subjects with known clinically significant medical conditions, including but not limited to: symptomatic coronary artery or peripheral vascular disease, malignancy within the past 5 years, except basal cell carcinoma; any condition compromising the function of those body systems that could result in altered absorption, excess accumulation or impaired metabolism or excretion of topiramate; subjects with history of attempted suicide/homicide in the past 12 months or suicidal/homicidal tendencies or judged clinically to be at serious suicidal/homicidal risk;
• Subjects who are pregnant or lactating;
• Subjects who have not observed the designated washout periods for any of the prohibited medications outline in this protocol;
• Subjects who in the opinion of the investigator should not be enrolled in the study because of the Precautions, Warnings, or Contraindications of the topiramate package insert.