An osteoblastoma is a benign bone tumor that most often occurs in the vertebral column and the long bones of the extremities. Paranasal sinus involvement is very rare. We report an osteoblastoma of the ethmoid sinus in a 13-year-old boy who presented with exophthalmos. CT demonstrated an expansile, lesion of the right ethmoid sinus, which caused the orbital contents to be compressed and deviated to the right. A mixture of ground glass opacity and dense bone was present. The less dense portions of the lesion showed intense enhancement on MRI; a signal void was present on all sequences in the densely sclerotic areas. Curettage was performed. Histologically, the lesion was composed of proliferating osteoblasts along with small trabeculae of woven bone and rich vascular fibrous stroma. The literature concerning occurrence of osteoblastoma in this unusual location is reviewed and discussed.