A 13-year-old female with a 4-year history of monostotic fibrous dysplasia had noticed a progressive proptosis of the right eye and diplopia on upward gaze for 4 weeks. A few years previously an incisional biopsy of the skull had verified the presumed diagnosis of fibrous dysplasia with recurrent bleeding into pathologic cystic bony structures of the skull. The patient was known to have craniofacial fibrous dysplasia with involvement of the frontal and intermediate cranial base, the posterior ethmoidal labyrinth, and the sphenoidal and maxillary sinuses. Eye examination showed a reduced visual acuity in the right eye without defects of the visual field. MR imaging showed a fluid-filled cystic cavity in the orbital frontal bone pushing the globe downwards. Four months later she developed similar symptoms on the other side while proptosis of the right eye was regressive. T2-weighted MRI revealed a large fluid-filled cystic cavity with a fluid-fluid level in the upper part of the left orbit. It is concluded that follow-up studies can be easily performed by MRI without additional exposure to radiation. The total extent of osseous involvement can be determined. Thus, MRI may be helpful in deciding between operative or conservative therapy.