Large accumulations of clear fluid in the cranial cavity, overlying, compressing, displacing, or replacing limited regions of the cerebral hemispheres, are a heterogeneous group of most unusual spaceoccupying lesions. According to their apparent location in the subdural or subarachnoid spaces and possible aetiology (focal inflammatory processes, congenital anomalies, trauma of meninges), these fluid-filled cavities or cysts, usually walled off by arachnoidal adhesions, have been described by Abbott (1933), Claude (1933), Okonek (1938), Soule and Whitcomb (1946), Taveras and Rouschoff (1953), and Zehnder (1938) under a variety of terms. Even though considerable confusion still exists in regard to the location and actual pathogenesis of the arachnoidal cysts, there is general agreement as to the origin of the cystic fluid of which the macroand microscopic appearances and chemical contents have been found to be quite similar to those of the cerebrospinal fluid. Opinions are also agreed on the potential pressure effects, manifested in particular cases of large collections of fluid, by localized thinning and bulging of the skull, and signs of brain compression. Although cranial enlargement, together with a subdural accumulation of fluids, was mentioned by Davidoff and Dyke as early as 1938, the first to draw attention to the close relationship that exists between large arachnoidal cysts and the associated localized thinning and bulging of the skull was Childe (1953). The latter author described as 'chronic subdural hygroma' eight such cysts and emphasized their frequent localization in the middle fossa. This particular type of arachnoidal cyst has, as a rule, a prolonged and relatively benign course, is clinically silent, and usually produces few, if any, symptoms in spite of insidious but progressive enlargement. As these cysts are most prevalent in children, in whom clinical evaluation is often difficult, their diagnosis is frequently obscure for many months or years and some cases may even escape detection altogether. At times, because of size and clinical behaviour, they may produce similar effects to those noted with any slowly expanding lesion and the correct nature of the disease process is, therefore, made either at surgery or incidentally at post-mortem examination. As we have recently had the opportunity to treat a young patient harbouring such a huge subarachnoidal cyst, and because of the paucity of the clinical experiences recorded, the report of the clinical and operative features of this case is presented as material which may shed some light on the controversial problem of the origin and pathogenesis of this condition.