A 57-year-old woman was admitted to Kakeyu Hospital complaining of paresis of the left upper and lower extremities which suddenly developed three years ago. Neurological examination revealed spastic paresis of the left upper and lower limbs without facial and lingual paresis. The reflexes were abnormally brisk on both sides but they were more remarkable on the left side, which also showed Babinski's sign. Sensitivity to light touch and vibration was moderately decreased and sensitivity to pinprick and joint position was minimally decreased in the left upper and lower extremities. No cerebellar sign was observed. Needle EMG disclosed large motor units with an amplitude of 4-5 mV and a duration of 8 msec in the light half of the tongue during weak contraction. MRI using a 0.5-T superconducting magnetic resonance unit detected a small, wedge-shaped infarction in the anterior medial portion of the medulla just below the pontomedullary junction. Somatosensory evoked potentials (SEPs) after median nerve stimulation with a non-cephalic reference were recorded. After stimulation of the left side, the scalp-recorded P13 was recognized at the normal latency, but the later components, N16 and N18, were apparently absent. On the other hand, SEPs in another case with thalamic hemorrhage revealed normal N16 potential with absence of N18 on the affected side. From these SEP findings and the reports on SEPs in lesions of the brain stem or thalamus, it was suggested that P13 is abnormal in lower medullary lesions and is preserved in upper medullary lesions, and that N16 is abnormal in brain stem lesions and is preserved or changed a little in thalamic lesions.