ABSTRACT
Since the arachnoid cyst was first described by Bright in 1831, its natural histroy and exact incidence of arachnoid cyst associated with chronic ubdural hematoma were not well understood. Authors analyzed 4 cases of arachnoid cysts in middle cranial fossa, associated with chronic subdural hematoma. So, the following results were obtained : 1) Headche and vomiting were presenting symptoms and its symptomas were due to chronic subdural hematoma rather than that of arachnoid cyst 2) Trauma, though trivial, may have triggered the development of chronic subdural hematoma 3) All cases showed bony abnormality on plain skull X-ray. 4) CT scan is very useful for the demonstration of shape, extent of lesion, membrane enhancement, and communication between hematoma and arachnoid cyst. 5) Every case was type II arachnoid cyst. 6) Chronic subdural hematoma was most frequent intracystic type, showing rupture into the arachnoid cyst. 7) Sugery treating both the chronic subdural hematoma and arachnoid cyst produced good result.