The authors report 4 cases of ossification of the posterior vertebral ligament. In 3 cases, the cervical spine was involved (mainly C4-C6), two included thoracic lesions. Three cases showed signs of spinal cord compression. Only one case was operated on. The authors review the literature. The authors discuss the classification, and the etiology, together with the possible relationship between the disease and ankylosing hyperostosis. Finally the pathogenesis is studied: the authors believe that spinal ischemia plays a very important role.