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Osteoblastoma is a benign bone tumor which commonly occurs in the vertebral column and the long bone of extremities, although the paranasal sinus involvement is very rare. We are reporting a case of benign osteoblastoma of the ethmoid sinus extending maxillary sinus and nasal cavity in a 66-year-old man who presented with progressive exophthalmos and intractable epiphora. The tumor was successfully removed via extended medial maxillectomy through lateral rhinotomy approach. Histologic features showed the central zone of edematous vascular fibrous tissue with scattered trabeculae of osteoid surrounded by the outer layer of dense mature bone. A differential diagnosis is necessary for various kinds of fibro-osseous lesions such as osteoid osteoma, ossifying fibroma and fibrous dysplasia. (Korean J Otolaryngol 2002;45:296-9)