Somatosensory evoked potentials were after double stimuli on the posterior tibial nerve for the intraoperative monitoring in 58 thoracolumbar or lumbar surgeries. The first cortical positive potential P38 and P'38 after the second stimulus was recorded after double stimuli. Transient decreased amplitude more than 50% or missing potential of P'38 was found in 11 cases. These P'38 changes occurred in five procedures of retracting the nerve roots or dural sacs, three cases of reducing of thoracolumbar fractures, and two procedures of transpedicular screwing. No causative procedure was found in one case. In those 11 cases, 5 showed no abnormality in P38 potentials and 6 cases showed less decreased amplitude in P38 than in P'38. Consequently, P'38, which was evoked by the second stimulus, can be used for the intraoperative monitoring in the thoracolumbar and lower lumbar surgery.