The aetiopathology, clinical features and treatment of arachnoid cysts (AC) are still extremely controversial topics. The posterior fossa is the second most common site of these lesions, since they are often detected in the cerebellar or cerebellar-cistern region. Despite this, almost all the cases of non-surgical reduction of an AC reported in literature concern cysts of the middle fossa, whereas only two cases of spontaneous reduction of a posterior fossa AC has been described in literature. A complete review of the literature regarding this topic has been collected and discussed. The authors present a case of spontaneous reduction of an arachnoid cyst situated along the midline of the posterior cranial fossa with regression of all neurological symptoms. This 43-year-old man presented increasing nuchal headache with vomiting and nausea, simulating subarachnoid haemorrhage. CT and MRI documented an arachnoid cyst along the midline of the posterior fossa compressing the mesencephalon. The patient did not present any family history of this pathology nor had suffered head trauma. The patient was only given analgesics to relieve painful symptoms. After 30 days, spontaneous resolution of all the neurological symptoms took place. A 2 months, MRI control documented marked reduction of the cyst without any evident brain compression. The feasibility of a wait and see policy in such cases is discussed.