IN SPITE of the increasing number of cases of optochiasmatic arachnoiditis reported in the literature and the general recognition of such a condition at operation, the etiologic agent in many instances remains obscure.1 Optochiasmatic arachnoiditis is defined as an abnormal thickening of the arachnoid in the region of the optic nerves and the optic chiasm. The process frequently extends to neighboring structures, in particular to the region of exit of both third nerves, and may reach the caudal portion of the pons, in which case the term basilar arachnoiditis is more appropriate. In the older literature the same condition goes under the name of chronic basilar meningitis.Fine films of arachnoid are normally present, forming the chiasmatic cistern. In cases of optochiasmatic arachnoiditis there are solid bands of the arachnoid, which insert either on the optic nerves or on adjacent structures, such as blood vessels. Or there are "aprons" of thickened arachnoid, hiding completely from view the optic nerves and chiasm.The condition gives rise to the chiasmatic syndrome of primary atrophy of the optic nerve and has been considered an indication for sur¬ gical intervention. Other causes producing a similar syndrome, according to Cushing,2 are suprasellar meningioma, pituitary adenoma, craniopharyngioma, glioma of the chiasm and suprasellar aneurysm.Optochiasmàtic arachnoiditis has a varied origin, which in most cases is not known. Vincent and his collaborators,1 who seem to have had the largest operative experience with this condition, give the follow¬ ing causes of its development: (1) infections of cavities of the face, (2) cerebromeningeal infections, (3) trauma and (4) rare and poorlyunderstood causes.