16 patients and 4 adult cases of small-or medium-sized middle fossa arachnoid cyst were subjected to various neuroradiological investigations and their results were analyzed in terms of possible mechanisms of cyst expansion. Based on the results of the investigations which strongly suggested that the cyst is an expanding lesion, all cases were surgically treated by the same techniques of craniotomy, excision of the outer cyst membrane, followed by a cystoperitoneal shunt, resulting in complete disappearance of the cyst concomitant with reexpansion of the surrounding brain and marked improvement in the clinical pictures of the patients.