Existing data suggest anterior cingulate cortex (ACC) plays a role in autonomic regulation. In persons with posttraumatic stress disorder (PTSD), autonomic regulation appears impaired and smaller mean ACC volume has been reported. This study examined relationships between ACC volume and the magnitude of respiratory sinus arrhythmia (RSA) in 77 U.S. combat veterans at rest, 40 of whom met criteria for PTSD. RSA magnitude did not differ in combat survivors with and without PTSD, which contradicts studies comparing civilians with PTSD to nontraumatized controls. RSA magnitude was positively correlated with right but not left hemisphere ACC volume. This finding was statistically independent of the presence or absence of PTSD.