Ten patients with selective impairment of either position sense or vibration sense were studied with somatosensory evoked potential (SEP). Five patients with spinal cord lesion (three with MS, one with spinal cord tumor and one with spinal cord injury) lost the vibration sense below the iliac crests without impairment of the position sense. However, five patients with cerebral vascular lesions involving thalamus unilaterally showed severe impairment of position sense, though there was no asymmetry as to the vibration sense. In all these cases with spinal and cerebral lesions, SEPs showed abnormalities in the distributions where the position sense was impaired and were not related to the impairment of vibration sense. Our study indicates that SEP is much better correlated with the position sense than with the vibration sense at any lesion level.