A 43-year-old female presented with persistent headache and dizziness which had first occurred two years earlier. The physical and neurological findings at admission were unremarkable, though plain radiography revealed the presence of a dense calcified mass in the left frontal area, and CT showed that a homogeneous high-density nodule was attached to the inner surface of the left frontal skull. The hard bony mass found and excised during surgery was shown at histopathologic examination to be a subdural osteoma. We describe the clinicopathologic findings of this entity and discuss the radiological features which suggest its subdural location.