We experienced a case of osteoid osteoma in thoracic vertebra accompanied with herniated nucleus pulposus during the management of back pain. A 32 year old male patient with herniated nucleus pulposus complained of back pain and radiation to right leg. Lumbar epidural block with 1% mepivacaine 5 ml was performed for pain control and it relieved the radiating pain. However patient continued to experience severe exacerated back pain at night which responded to aspirin. Therefore we performed further examination for existence of disease of the spine and diagnosed osteoid osteoma in the right pedicle of T12 vertebra. In conclusion, we recommend physicians when evaluating patients with back pain to be congnizant of possible existence of neoplastic disease of the spine and incorporate it in differential diagnosis.