Conditions that simulate brain tumors so closely as to be mistaken for them are by no means rare. They have been classified by some as "pseudotumors." Certain of them demand prompt surgical relief for the same reasons that tumors do, if vision is to be preserved and discomforts alleviated. Among them are certain forms of chronic or subacute arachnoiditis, to which attention is to be called in this communication.

The condition of chronic arachnoiditis has been designated variously as (1) "chronic serous arachnoiditis," a generalized process, and (2) "chronic circumscribed" or "adhesive" or "cystic arachnoiditis," when the condition has been localized. The latter process appears definitely to be the sequel of a meningo-encephalitis that has left a more or less circumscribed area of adhesions between the arachnoid and the underlying patch of atrophied cortex, with the formation of an arachnoid or pseudocyst. Similar lesions have been encountered both at operation