The authors present CT and MRI of a patient with an extremely large arachnoidal 
cyst (Galassi classification type III). The cyst extended from the base of the 
skull, posterior to the brain stem, on the base of the temporal lobe over the 
complete convexity of the left hemisphere. The cyst consisted of multiple 
compartments with intracystic septa and was accompanied by a chronic subdural 
hemorrhage in the compartments. After contrast agent application, the typical 
characteristics of chronic subdural membranes were found. Besides bony 
deformities, a thinning of the inner table was found. The patient underwent 
craniotomy for evacuation of the hemorrhage and fenestration of the septa while 
he was free of symptoms. This is a remarkable case proving that chronic local 
intracranial pressure does not inevitably lead to neurologic symptoms or 
intellectual disabilities.