Many patients with ossification of the cervical posterior longitudinal ligaments have stiff and wrinkled skin. This reflects abnormal metabolism in connective tissue during the ossification of the ligament. Collagen, a major component of the extracellular matrix of connective tissue, mainly consists of Type I and Type III. It has been reported that the ratio of collagen type I/III affects the elasticity and fragility of connective tissue. It has also been reported that the cross-linkings between collagen molecules influences the stiffness of skin. Therefore, we investigated the skin of patients with OPLL and used SDS-polyacrylamide gel electrophoresis to examine the ratio of collagen type I/III. Results indicated a significant decrease of the ratio of collagen type III to type I.