Thirty-nine cases of primary intracranial arachnoid cyst in the elderly, including one case of our own, were analyzed. Clinical characteristics in these patients were as follows. (1) The number of patients decreased with age, but there were no differences according to sex. (2) Clinical manifestations were similar to those of chronic subdural haematoma or normal pressure hydrocephalus including dementia, urinary incontinence, and hemiparesis. General symptoms such as headache and seizures were also present. (3) Surgery was performed in most patients with generally good outcome regardless of operative procedures (capsular resection vs. shunt). (4) In some cases of advanced age, disease manifestation may have been due to slight head injuries.