One hundred twenty-three cases of cervical spondylotic and ossification of the posterior longitudinal ligament (OPLL) myelopathy cases with long tract signs subjected to surgical treatment were studied to identify the most important factors having an influence on postoperative outcome using evoked spinal cord potentials (ESCP). Disappearance and positive wave changes of these potentials at the level of responsible lesions and slow conduction velocity under 40 m indicated an unsatisfactory outcome. Localized-lesion cases diagnosed by ESCPs had excellent results, significantly more so than extensive-lesion cases, regardless of operative methods. In 123 cases, 76% were found to have localized lesions, while the other 24% showed extensive lesions. Concerning the difference between CSM and OPLL, 54% of OPLL and only 14% of CSM demonstrated extensive lesions.