 Participants with 911-related or military service in the Iraq War-related PTSD are assigned to one of two programs, following an initial assessment. Both programs include Virtual Reality Exposure Therapy VRET for the treatment of PTSD as well as widely-used standard cognitive-behavioral therapy techniques. The VRET is comprised of ten ninety minute sessions which involve mental imagery as well as virtual reality presentations delivered by a special display worn over the head and eyes. The virtual reality simulations consist of either images of the World Trade Center and the events of September 11, 2001 or multiple scenarios common to military personnel assigned to Iraq. One group of participants will receive the medication cycloserine seromycin and one group will receive a placebo drug like a sugar pill. Participants take a dose of medication only on the days that they come to the laboratory for the Virtual Reality Exposure VRE sessions, and 90 minutes prior to the VRE sessions. Some subjects in the study will receive a 100 mg dose of cycloserine, and other subjects in the study will receive the placebo sugar pill. This will be determined randomly as by the flip of a coin. Neither the patient nor the treating clinicians will know which pill that patients are receiving; however, this arrangement may be changed if medically necessary. Patients are briefly assessed for well-being at every session, and are reassessed following session three, session six, and session ten for changes in symptomatology. At the completion of treatment and again six months later, the principal investigator or one of her associates will reinterview participants and have them complete some questionnaires. 