PURPOSE: To evaluate the relationship between spinal instability and ossification of the posteriorlongitudinal ligament(OPLL). MATERIALS AND METHODS: In 70 patients(M:F=45:25, mean age=53years) diagnosed as OPLLon the bosis of with surgical operation field findings and radiological evaluation[ plain film(n=70), CT(n=64),MRI(n=55) ], involved levels were the cervical spine(n=32), lumbar spine(n=23), and both the cervical and lumbarspine(n=15). Spinal instability was radiologically diagnosed as horizontal displacement of one vertebra by anotherof more than 3.5mm, or a difference in rotation from either adjacent vertebra by more than 11degree in lateralcervical spine and a difference of more than 1.5mm from the posterior body margins to the point of intersection oftwo lines drawn parallel to the opposing segmental endplate in extension lateral lumbar spine. We divided OPLLinto group I(continuous, segmental, mixed) and group II(retrodiscal), and compared spinal instability in these twogroups. RESULTS: In cervical OPLL, group I comprised 33 cases and group II 14. In group I, spinal instability wasnoted in 8/33 cases(24%)) or 10/123 segments(8.1%). Spinal instability in group II, on the other hand, was foundin 13/14 cases(93%) or 17/26 segments(65%). Ossification occurred at the retrodiscal level in 37 cases, byt incase was continuous. In group II, spinal instability was found in 25 of 37 cases(69%), oe in 29 of 55segments(53%). CONCLUSION: Compared to other types of OPLL, the frequency of retrodiscal OPLL in association withspinal instability was high. Spinal instability may thus be the most important cause of retrodiscal OPLL.