Passive P300 (PP300) brain evoked responses were compared in 10 normal subjects and 13 subjects with severe traumatic brain Injury (TBI). Electrophysiological responses in patients with TBI were slower for peaks designated as N250, P300, and N400 and generated lower voltages for amplitudes N250-P300 and P3OO-N4OO. A significant anterior-posterior (Fz'-Pz') voltage difference was found in normal subjects but not in subjects with TBI. Tills difference may be both a marker of severity of brain impairment as well as recovery potential in patients with TBI. The finding of a PP300 response in patients with severe TBI, most of whom were in coma or near-coma, raises questions about the “cognitive” nature of the P300 response. Its presence suggests antecedent pre-cognitive subcortical or cortical activity such as primitive orienting to unexpected stimuli that affects higher level cognitive responses when head injury is not severe and such responses are possible.