Existing data suggest anterior cingulate cortex plays a role in autonomic regulation. In persons with posttraumatic stress disorder, autonomic regulation appears impaired, and smaller mean anterior cingulate cortex volume has been reported. This study examined relationships between anterior cingulate cortical volume and the magnitude of respiratory sinus arrhythmia at rest in seventy-seven U.S. military survivors of combat, forty of whom met criteria for posttraumatic stress disorder. RSA magnitude did not differ in combat survivors with and without PTSD, contradicting studies comparing civilian PTSD to non-traumatized controls. RSA magnitude was positively correlated with right but not left anterior cingulate volume, a finding statistically independent of the presence or absence of PTSD.