To evaluate certain aspects of cognitive processing in posttraumatic stress disorder (PTSD), brain event-related potentials, primarily P300, to traumatic stimuli were recorded. 20 Israeli veterans with PTSD of combat origin and matched controls were studied. In a modification of the oddball paradigm, subjects were asked to react to target pictures of traumatic combat-related subjects while ignoring non-target pictures of irrelevant subjects. PTSD patients reacted with an accentuated P300 and prolonged reaction times to the traumatic pictures. In addition, P300 and reaction times to the target pictures were prolonged in PTSD patients as compared to controls. Event-related potentials (P300) to traumatic stimuli can serve as an objective and accurate brain parameter for characterization and diagnosis of PTSD. In addition, it contributes to understanding the pathogenesis of PTSD as a specific impairment in information processing.