In this paper a case of monostotic fibrous dysplasia in the left parietal bone is described. This 51-year-old female was admitted to our hospital for a head injury on January 12, 1983, and a flat, painless hump (7 X 7cm) was incidentally found in the left parietal region. Plain x-ray film of the skull showed a multicystic lesion with slightly sclerotic margin in the left parietal bone, and outward bulging of the outer table without destruction of the inner table in tangential view. CT scan at the level of bone window clearly demonstrated the same abnormality and intradiploic mass separated by some bony septums. Angiography revealed no positive findings, but RI bone scan (99m Tc) showed an abnormal uptake in this lesion. On February 1, 1983, operation was performed to verify the nature of the lesion and for relief of the left-sided headache. Post operative course was uneventful. Histological findings of this mass were of inactive fibrous dysplasia with fibrous tissue replacing the normal bone and surrounded by island or scorlled edge of bones. Monostotic fibrous dysplasia in the cranial vault is rare. It is often possible to make diagnosis in this disease on the clinical course, plain x-ray film and CT scan showing cystic findings and outer table bulging, and RI scan demonstrating abnormal uptake.(ABSTRACT TRUNCATED AT 250 WORDS)