Reportedly, ossification of the posterior longitudinal ligament (OPLL) usually involves the cervical spine and often accompanies other ligamentous ossification such as diffuse idiopathic skeletal hyperostosis (DISH). It is considered serious because it sometimes causes severe radiculomyelopathy; however, the present study, based on a fixed population sample, revealed that OPLL of the thoracic spine is nearly always asymptomatic. The prevalence of thoracic OPLL was 0.6%, with three times as many women as men being affected, compared with cervical OPLL which occurs predominantly among men. No marked radiculomyelopathy was observed, nor definite evidence of neurological involvement due to thoracic OPLL. DISH was rare, especially among women.