A case of monostotic fibrous dysplasia in the anterior skull base, which presented with visual disturbance during pregnancy, is reported. A 32-year-old female was referred to our department for examination of the progressing right visual disturbance in the third trimester of her second pregnancy. She had experienced the same episode in her first pregnancy, recovering from it after delivery. This time, however, the visual acuity did not change after delivery. Plain craniogram showed sclerotic changes in the right sphenoidal ridge and the right frontal skull base. CT scan showed an isodense mass which was enhanced by contrast medium in the right sphenoidal sinus. Angiography demonstrated no positive findings. RI bone scintigram using 99 m Tc-MDP revealed an abnormal uptake in this region. The patient was operated on in two stages. The first operation was transsphenoidal removal of the tumor in the sphenoidal sinus. The pathological diagnosis was fibrous dysplasia. Transfrontal decompression of the right optic canal and ophthalmic artery was performed at the second operation. The tumor was totally removed and the decompressed orbital roof was reconstructed using an alumina ceramic plate. Visual acuity gradually improved in the follow-up study. To our best knowledge, only one case of fibrous dysplasia with growth during pregnancy have been reported; it was monostotic fibrous dysplasia of the maxilla. From the clinical course of our case, it is suggested that the pregnancy influenced growth of the monostotic fibrous dysplasia; possibly by way of hypothalamic hormonal factor.