This study provides a 12-14 week treatment for posttraumatic stress disorder related to the World Trade Center Attacks of September 11, 2001. Every patient will receive weekly individual therapy consisting of cognitive-behavioral treatment, including exposure therapy. In this treatment study, exposure will be enhanced by virtual reality and each patient will be randomly assigned to receive either D-Cycloserine or a placebo pill that is taken only on the day of the treatment when the exposure will be done for a total of 10-12 times. D-Cycloserine is a broad spectrum antibiotic that recently has been implicated as a cognitive enhancer which may speed up the learning process that occurs during exposure and/or make the gains more lasting.