Radial bone mineral density (BMD) in the distal one sixth and distal one third of the left radius was measured by single-photon absorptiometry (SPA) in patients with ossification of the cervical posterior longitudinal ligament (OPLL). Twenty-three OPLL patients (15 men and 8 women) with mean initial age of 60.1 years were studied longitudinally for mean follow-up period of 3.9 years. Based on roentgenologic findings, the patients were divided into two groups, those with advancing OPLL and those with non-advancing OPLL. The final value of distal 1/6th BMD in the advancing group was significantly lower than its initial value, but no change of the value of the distal 1/3rd BMD was observed in either group. The final value of distal 1/6th BMD/distal 1/3rd BMD ratio in the advancing group was significantly decreased compared with its initial value. These results suggest that, in the advancing OPLL group, trabecular bone mass decreased while cortical bone mass remained unchanged.