Multiple dural sac and spinal cord indentations are often observed on MRI in cervical spondylotic myelopathy. However, it is rare that all of the indented levels contribute to clinical symptoms. Pathological changes in cervical compression myelopathy have previously been reported. Still the critical degree of spinal cord compression needed to induce pathologic changes is unknown. To clearfy this matter the relationships between the spinal cord as well as the dural sac indentations, and pathological changes of the cervical spinal cord were investigated in cadavers. Sixty-eight cadavers were used for this study. The dural sac and the spinal cord were observed especially regarding presence of indentations. The spinal cord was removed from the specimens in order to perform histopathological examination. The indentations of the dural sac and the spinal cord were observed at C4/5, C5/6, C6/7 and C3/4 intervertebral levels in order of incidence. However, all of the dural sac indentations were not correlated with spinal cord indentations. Pathological changes in the spinal cord were observed in two specimens with less than 30% of the AP compression ratio. One specimen with 20.9% in the AP compression ratio had remarkable histopathological changes. In the other specimen with 29.6% in the AP compression ratio, diffuse demyelination was distributed in the lateral white matter. The results indicate that the critical degree of the AP compression ratio is 30% to induce histopathological changes in the spinal cord. If a spinal cord indentation in the patient with cervical spondylotic myelopathy on imaging, i.e. MRI, show less than 30% in the AP compression ratio, the clinical symptoms, i.e. numbness, tickling and paresthetic pain, may not be improved after the surgery because of some histopathological changes in the spinal cord.