Osteoblastoma is a rare bone tumor that involves abnormal bone and osteoid tissue formation. It was classified as an intermediate（ locally aggressive） tumor by the WHO in 2013. We describe a case of mandibular osteoblastoma in a 9-year-old boy whose chief complaint at initial consultation was facial asymmetry. Radiographic examination showed a well-defined radiopaque region measuring approximately 26 × 21 mm in diameter that was partially surrounded by a radiolucent area. In the right side of the mandible, the second premolar was impacted by the tumor. Biopsy findings suggested an osteoblastic neoplasm with an MIB-1 index of about 1%. Based on these findings, we planned to perform curettage of the tumor. A histologic examination of the excised specimen revealed it was mainly composed of abundant connective tissue showing active angiogenesis and osteoid bone covered with osteoblast-like cells, thus confirming our initial diagnosis. The postoperative course was uneventful. One year 8 months after the operation the impacted tooth had erupted spontaneously. This case suggests that it might be feasible to preserve permanent teeth impacted by an osteoblastoma.
