Preclinical and clinical studies demonstrate a hyperactivity of the norepinephrine system in patients with posttraumatic stress disorder (PTSD). a(2) adrenergic agonists have been shown to ameliorate symptoms of PTSD, likely because of their ability to dampen noradrenergic tone. This study tests the ability of the a(2) adrenergic agonist, guanfacine, to reduce the symptoms of PTSD. Patients with chronic PTSD were randomized (1:1) to an 8-week double-blind, placebo-controlled treatment of guanfacine followed by a 2 month open label extension phase. Patients were maintained on their stable doses of allowed antidepressants during the trial. Efficacy was measured by the following assessment scales: Clinician Administered PTSD Scale (CAPS), Montgomery Asberg Depression Rating Scale (MADRS), Clinical Global Impression-Severity (CGI-S), Clinical Global Impression-Improvement (CGI-I), and Davidson Trauma Scale (DTS, self-report). There were no significant differences in the drug versus placebo responses for the clinician-administered or patient self-report outcome measures in this small sample of predominantly male combat veterans with PTSD. However, the medication was well tolerated. Similar to previous findings, this small pilot study failed to show differences in the response to guanfacine versus placebo in a small sample of predominantly male combat veterans with PTSD.