OBJECTIVE: To evaluate the efficacy of divalproex for the treatment of posttraumatic stress disorder (PTSD) hyperarousal symptom cluster. METHOD: Under double-blind conditions, 85 US military veterans with PTSD were randomized to treatment with divalproex or placebo for 8 weeks. All patients who received at least 1 dose of medication and 1 postbaseline assessment (n = 82) were included in the efficacy population. The primary outcome measure was the hyperarousal subscale of the Clinician-Administered PTSD Scale. RESULT: There were no significant intergroup differences in primary or secondary end points. The final mean (SD) divalproex dose and serum valproic acid level were 2309 ± 507 mg/d and 82 ± 30 mg/L, respectively. CONCLUSIONS: Divalproex monotherapy was not effective in the treatment of chronic PTSD in predominantly older male combat veterans. Further study is needed to determine the efficacy of divalproex in the management of PTSD in women or civilians or in combination with antidepressants. © 2008 Lippincott Williams & Wilkins, Inc.