The thoracic ossification of ligamentum flavum (OLF) is a disease that produces spastic paraparesis, and there are various factors that may affect the surgical outcome of thoracic OLF patients. The authors of this study treated 19 of these thoracic OLF patients from 1998 to 2002, and retrospectively reviewed the patients' age, sex, symptom duration, involved disease level, preoperative clinical features, neurological findings, radiological findings, the other combined spinal diseases and the surgical outcomes. There were excellent or good surgical outcomes in 16 patients, but 3 patients did not improve after thoracic OLF surgery: this included 1 patient, whose motor function worsened after decompressive thoracic OLF surgery. The favorable contributing factors of surgical outcome in thoracic OLF are a short preoperative symptom duration, single-level lesion, and unilateral lesion type on CT axial scan. On the contrary, the poor prognostic factors are beak type lesion and intramedullary signal changes on T(2)-weighted sagittal MRI. The complete preoperative evaluation including radiologic findings will provide valuable aid in presuming the surgical outcome for the thoracic OLF patients.