We report a case and discuss clinical features and radiological differential diagnosis of dorsal idiopathic extramedullary arachnoid cyst associated to a caudal syringomyelic cavitation. The aim of this article is to review the current literature on a rare topic as idiopathic intradural extramedullary arachnoid cyst with associated syringomyelia. We emphasize the fact that correct diagnosis and adequate treatment, which we believe is microsurgical fenestration of the cyst into the subarachnoid space, may lead to disappearance of syringomyelia and complete resolution of preoperative spinal cord compression symptoms.