The pathogenesis of ossification of the posterior longitudinal ligament (OPLL) remains to be elucidated, though etiologic factors for OPLL have been identified. High levels of serum retinol and retinol binding protein (RBP) have been observed in patients with diffuse idiopathic skeletal hyperostosis (DISH). OPLL is often associated with DISH. In this study, the levels of serum retinol and RBP were determined in 70 patients with OPLL in the cervical spine, and compared with those in normal subjects. Bone metabolic markers of serum intact osteocalcin, urinary pyridinoline and deoxypyridinoline were examined as well. Among female patients, level of serum RBP was significantly higher in those in their 60's, and those with mixed type OPLL. Level of serum RBP was significantly higher in both sexes, and retinol was exhibited higher in female patients, if they were associated with DISH. Patients with OPLL exhibited no abnormal bone metabolic marker levels. These findings suggest that vitamin A may play a role in the development of OPLL.