No abnormalities in magnetic resonance images were recorded in patients with posttraumatic stress disorder other than an increased incidence (50%) of a small cleft in the callosal-septal interface, a cavum of the septum pellucidum. A similar grade of cavum was obtained in 14% of normal volunteers matched for age, socioeconomic background, and military experience. The cavum is believed to have antedated the disorder and is conceived to be a neurodevelopmental aberration. The possibility that the cavum is a marker of vulnerability to stress in psychopathology is discussed.