Arachnoid cysts are intra-arachnoid collections of cerebrospinal fluid. Arachnoid cysts of the posterior fossa are rare lesions that are considered to be mostly congenital in origin. Arachnoid cysts of the posterior cranial fossa may manifest themselves in several different ways. When they are symptomatic, headache, gait disturbance, nausea, vomiting, focal neurologic signs, dizziness, and seizures are most common in the patients with increased intracranial pressure. Increased intracranial pressure is caused by the ball-valve mechanism of the cyst's membrane that communicates with subarachnoid space or arachnoid cells and contains specialized membranes and enzymes, which have secretory activity. A postsurgical arachnoid cyst in the posterior cranial fossa has doubled and slitlike arachnoid membrane that supports our knowledge about the underlining mechanism is reported.