In 1985 and 1986 the Department of Neurosurgery of the University of Berlin treated two cases of arachnoid cysts of third ventricle, both of which were successfully dealt with surgically. They are reported in this paper. Arachnoid cysts from ventricular system and cisterns are rare. Up to now 46 cases have been reported in the specialized literature. The computerized tomography and magnetic resonance make diagnosis easier, as shown through the cases reported.