 This study proposes to evaluate the effects of D-cycloserine DCS combined with cognitive-behavioral treatment with exposure therapy in a sample of patients who developed posttraumatic stress disorder PTSD as a consequence of various traumas e.g., motor vehicle and accidents, burns and other injuries, combat, World Trade Center attack, etc.. 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 SNP Val66Met, predicts treatment response to PTSD. 