In 18 cases of temporal arachnoid cysts the etiology, clinical and radiographic findings, surgical treatment, and outcome are reviewed. Cysts of the middle cranial fossa are susceptible to trauma, which may cause bleeding either into the cyst or into the subdural space. CT or MRI scans are diagnostic in arachnoid cysts. In cases of intracranial mass lesion with displacement of the midline structures and increasing ICP, osteoplastic craniotomy is performed and the medial wall of the cyst is resected down to the tentorial notch, with opening into the basal cisterns. There were no operative or postoperative complications in 18 consecutive cases. However, one boy required a cystoperitoneal shunt 3 months later as a result of hydrocephalus following subdural hematoma and two other patients were also subsequently shunted. Asymptomatic arachnoid cysts are discussed with the respect to brain function and potential risks.