Ossification of the posterior longitudinal ligament of the spine (OPLL) is a newly recognized entity. As the incidence of this disease was exceptionally high in Japan, the Japanese Ministry of Public Health and Welfare instituted a special commission for the investigation of this perplexing disease; since 1975 this committee has performed an intensive study of 2100 patients with OPLL in Japan. An epidemiologic study was conducted by this group in Japan and in eastern Asiatic countries. Symptoms and disabilities caused by the disease were described. Roentgenographic findings were classified as continuous, segmental, mixed, or localized. OPLL at the thoracic and lumbar levels combined with ossification of the yellow ligament was described, and the risk of spinal cord damage as well as the importance of tomography and computerized tomographic scanning were stressed. No conclusions were reached concerning etiology, but common findings included a generalized hyperostotic tendency, a tendency for abnormal glucose metabolism, and low enteral calcium absorption. A relatively high hereditary occurrence was noted. Conservative and surgical treatment methods were described, with particular reference to spinal canal-widening operations.