The clinical features of 166 patients with ossification of the posterior longitudinal ligament are reported. Symptoms of cervical myelopathy develop and include difficulty walking and sensory and motor disturbances. Roentgenograms disclose dense ossification posterior to the vertebral bodies, and symtoms are most often observed when the ossification occupies more than 60% of the saggital diameter of the cervical canal. Posterior decompression is the most effective surgical treatment. Two patients with ossification of the posterior longitudinal ligament and ankylosing spinal hypertosis expired and necropsy was performed. The histologic features of the spine and spinal cord are described.