[en] Arachnoid cysts are benign congenital cavities arising in the subarachnoid space. Non-traumatic subdural effusion of cerebrospinal fluid is a rare complication requiring surgical treatment. Case Report: A 15-year-old boy was admitted to the hospital because of symptoms related to acute intracranial hypertension (headache and vomiting). The cerebral CT-scan revealed a subdural hygroma adjoining a voluminous Sylvian arachnoid cyst. Two arachnoid cysts were incidentally discovered 11 years before this dramatic complication. Moreover, the patient had suffered a cerebral concussion 2 years earlier, but interestingly did not develop cystic hemorrhage or rupture, contrary to numerous cases previously described in the literature. The location of the cysts and their regular follow-up did not allow foreseeing a cystic rupture. Hygroma evacuation was first performed after which a subdural peritoneal. shunting, using a programmable opening pressure valve, was implanted. Spontaneous rupture into the subdural space represents an unusual complication of arachnoid cysts. Clinical aspects, radiographic findings, pathogenesis and surgical management are described. It is important to point out that subdural hygroma or haematoma should never be excluded in the absence of trauma history, even in the case of small non-progressive cysts regularly supervised. (C) 2004 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.