Arachnoid cysts of the middle cranial fossa (Sylvian cysts), represent the most common type of intracranial leptomeningeal malformation. Among the 102 intracranial arachnoid cysts operated on at the authors' institution from January 1970 to August 1986, the 77 cases (75%) located in the middle cranial fossa are reviewed. The higher incidence in the first two decades of life (51 cases) as well as the marked predilection for the male sex (60 cases) and the left hemisphere (55 cases) are confirmed in the authors' experience. As for clinical presentation cranial deformities, symptoms of raised intracranial pressure and epilepsy constituted the most frequent features. In 13 patients a complicating lesion was associated: subdural or intracystic haematomas in 7 cases, subdural hygromas in 4 cases and, extradural haematomas in 2 cases. Based on the appearance at CT scan and the results at CT cisternography the authors proposed a classification into three basic types of increasing severity and different pathophysiologic conditions. All the patients underwent craniotomy, excision of the cyst walls and perforation into the basal cisterns. There was one postoperative death (mortality rate of 1.3%) due to meningitis. The remaining clinical results were gratifying in all three types of lesion; on follow-up CT scans the cysts of type I. and II. exhibited a steady tendency to reduction or obliteration while cerebral reexpansion seemed less evident in the third, most severe, type. The authors compare and discuss the options of radical open surgery versus shunting procedures.