The ossification of the cervical posterior longitudinal ligament (OPLL) is widely known and studied in Japan where a roentgenological incidence of 2.06% adults affected has been found. Data concerning the ossification of the lumbar posterior longitudinal ligament are few and occasional. An epidemiological survey on lumbar OPLL was performed by the authors in Matsumoto, Japan, on a total of 792 subjects, 554 of whom over the age of 35, by means of X-ray of the lumbar spine. Ossification of the lumbar posterior longitudinal ligament was detected in 23 subjects (2.9%), with no significant difference between males (3.0%) and females (2.8). Lumbar OPLL was absent in the 238 subjects aged less than 34; it was the most prevalent after the age of 45 (5.1% in males and 4.5% in females). The ossification developed in two ways: continuous ossified layer extending over several vertebrae; circumscribed ossification of the ligament corresponding to the level of the intervertebral disk (retrodiscal type). The result of this epidemiological survey showed a roentgenological incidence of lumbar OPLL of the same magnitude than that of cervical OPLL.