To study the genetic background in ossification of the posterior longitudinal ligament of the spine (OPLL), 1,030 relatives of probands with cervical OPLL in 347 families were investigated by observing their spinal x-ray films, and the following results were obtained. Ossification of the posterior longitudinal ligament was found in 26.15% of the parents of the probands and 28.89% of the siblings of the probands. These prevalences were significantly higher than in the general population. The segregation rate in the siblings corresponded neither with the hypothesis of autosomal dominant inheritance nor with that of autosomal recessive inheritance. The possibility of polygene inheritance was also ruled out, considering the risk of recurrence in the siblings of the probands. However, owing to the segregation rate in the siblings exceeding 0.25 and the high prevalence of OPLL in the parents, OPLL was suggested to be a disorder possibly controlled by autosomal dominant inheritance.