The authors describe their experience with 26 cases of thoracic myelopathy secondary to hypertrophic ossification of the spinal ligament (posterior longitudinal ligament and/or ligamentum flavum). The clinical manifestations of this condition and results of its surgical treatment are described. The commonest symptoms were numbness or tingling in the legs and feet and gait disturbance. Most of the patients with involvement of the upper thoracic spine showed typical features of thoracic myelopathy: that is, sensory and motor deficits in both the trunk and lower extremities, sphincter disturbance, and exaggerated tendon reflexes. Several patients with involvement of the thoracolumbar junction presented with atypical symptoms of thoracic myelopathy and were sometimes misdiagnosed and treated inappropriately. Surgical treatment, particularly laminectomy, was not always successful. Inconsistencies in the surgical outcome were caused by either operative complications or reversal of the initial improvement during the follow-up period. The results of anterior surgery for the condition were more favorable; however, use of this procedure was rarely indicated.