Arachnoiditis involving the optic nerves and chiasm has been reported previously by Heuer and Vail4 (1931), Craig and Lillie1 (1931), and Ryan7 (1943). Except for Davis and Haven's3 study of the arachnoid membrane (1931), the reported cases were clinical and surgical without microscopic study of the adhesion or cyst. The clinical concept of arachnoiditis does not appear to be clearly defined in previous reports, for Walsh8 stated that he was skeptical regarding its existence. The recognition of arachnoiditis is of great importance, since it frequently produces a chiasmal syndrome which simulates a suprasellar neoplasm or aneurysm. Further, with lysis of adhesions or release of cystic collections of fluid by surgery, many have obtained improvement of vision. The occurrence of a free subdural membrane with arachnoiditis of the optic nerves and chiasm after spinal anesthesia and meningitis has not been previously noted. The following case is,