Although the pathogenesis of ossification of the cervical posterior longitudinal ligament (OPLL) has not yet been clarified, it has come to be widely recognized that severe cervical myelopathy or radiculopathy is caused by OPLL. Fifty-three cases who were operated on for OPLL with myelopathy or radiculopathy in our clinic over the past 16 years were followed up. A recovery rate of approximately 70% was observed. Postoperative progressions of the ossification were observed among 75% of the cases of continuous and mixed type but seldom among those with segmental and other types. As causative factors for these postoperative progressions of the ossification, the authors would like to advocate biological, structural, and mobility-related elements. We concluded that in the ossified stage it is desirable to apply anterior decompression for the segmental and other type, posterior decompression for the continuous and mixed type, and, if necessary, two-stage combined decompression for the mixed type.