The authors report 46 cases of primary supratentorial arachnoid cysts. Thirty-two patients presented with symptomatic arachnoid cysts, which act as space-occupying lesions and were surgically treated by fenestration. 14 patients presented with a scarcity of symptoms; for this reason they were kept under control by periodical neuroradiological examination. After a long follow-up period (median 11 years), 97% of the operated patients presented neurological improvement and a decrease in the size of the cyst; none of the patients who were not operated on showed any variation in either neurological status or size of the cyst. The authors discuss their findings in the light of the published papers.