 This study proposes to evaluate the effects of D-cycloserine DCS combined with Virtual Reality exposure therapy in a sample of patients who developed posttraumatic stress disorder PTSD following either the events of September 11, 2001, or military service in the war in Iraq. In addition, this study hopes to determine whether a common human genetic single nucleotide polymorphism SNP in a growth factor, brain derived neurotrophic factor, BDNF Val66Met, predicts treatment response to PTSD. 