Posttraumatic stress disorder (PTSD) is characterized by intrusive, avoidance, and hyperarousal symptoms. This study was conducted to investigate the effectiveness of divalproex in reducing PTSD symptoms, depression, and anxiety in patients with PTSD. Sixteen patients with a DSM-IV diagnosis of PTSD at the Albuquerque VAMC outpatient PTSD treatment program received an open-label trial of divalproex. The patients were evaluated at baseline and at 8 weeks by a trained rater using the Clinician Administered PTSD Scale (CAPS), the Hamilton Rating Scale for Depression (HAM-D) and the Hamilton Rating Scale for Anxiety (HAM-A). Plasma valproate levels were measured at the 8-week post-treatment assessment. Three patients stopped the medications due to side effects. Intrusion and hyperarousal symptoms decreased significantly, while no significant change was seen in avoidance/numbing symptoms. Depressive symptoms, as measured by the HAM-D, unexpectedly decreased at post-treatment assessment. HAM-A scores also decreased significantly. Controlled trials are needed to further study the efficacy of divalproex in the treatment of PTSD.