Arachnoid intracranial cysts are benign lesions which until recently have been considered to be quite uncommon. They are benign congenital lesions usually discovered at an early age. We have recently treated two patients with advanced age and arachnoid cysts, unusual due to their location and extent. Both of them presented clinically with an organic dementia syndrome with some similarities to that seen in normal pressure hydrocephalus. Both recovered fully after surgery. We review the literature and try to emphasize the clinical differences between arachnoid and the more rare neuroepithelial cysts.