Severe traumatic brain injury (TBI) patients in a vegetative state were exposed to passive P300 (PP300) evoked potential tests under two conditions: two auditory tones (unimodality condition), and a flash and auditory tone (bimodality condition). A third non-P300 condition using a single repetitive auditory tone was also presented. Patients produced PP300 responses under all three conditions, even though the severity of their clinical condition did not allow them to respond to even simple commands. No peak latency differences were found. PP300 amplitude was significantly larger under the bimodality stimulus condition than either the unimodality or non-P300 condition. The PP300 amplitude under the unimodality condition, in turn, was larger than the P300-like response in the non-P300 condition. This replicates earlier findings with normal subjects. PP300 responses appear to be a tool that might find utility in evaluating TBI patients. Results raise questions about the neuropsychological/neurophysiological nature of the PP300 response.