Clinical assessment of the unresponsive patient is limited to examination of brainstem reflexes and simple motor responses to stimulation. It is thus difficult, especially if brainstem functions are intact, to give early, accurate prognostic information on comatose patients. Neurochemical tests and imaging have not been validated and have significant limitations. Electrophysiologic investigations provide a window into cerebral function and are tested, clinically useful, safe, available, and inexpensive. Persistent abnormalities of brainstem auditory evoked potentials and short-latency somatosensory evoked potentials reliably indicate the likelihood permanent vegetative state or death. Conversely, the presence of "cognitive" event-related brain potentials (e.g., P300 and mismatch negativity) reflects the functional integrity of higher-level information processing and, therefore, the likelihood of capacity for cognition. A combined clinical and electrophysiologic approach provides optimal prediction of outcome and level of disability.