Somatosensory evoked potentials, brain-stem auditory evoked potentials and electroencephalograms were obtained from 9 patients with the diagnosis of 'locked-in' syndrome. No pattern of evoked potential abnormality was specific to this syndrome, with findings ranging from bilaterally normal to unilaterally or bilaterally absent. The evoked potential studies complemented radiographic findings in defining the extent of the lesion and revealed that a portion of the pontine tegmentum was usually involved. Pathology from 2 patients corroborated the findings of the evoked potential studies. The value of evoked potential studies of patients with locked-in syndrome is to provide early objective evidence of brain-stem involvement independent of the clinical examination, EEG and radiographic studies.