It is difficult to identify the exact localization, subarachnoid space or subdural space, when hypodense extracerebral fluid collection is seen on initial or follow up brain CT scan. In cases with extracerebral fluid collection, CT findings are nonspecific and it is not always possible to confirm the exact location. We reviewed CT findings of 31 cases with hypodense extracerebral fluid collection for the purpose of establishing diagnostic criteria between subarachnoid space and subdural space. The findings were classified to three types according to bilaterality and configuration of cortical sulci ; type I, widening of bilateral extracerebral space and cortical sulci widening, type II, widenign of bilateral extracerebral space and cortical sulci obliteration, and type III, widening of unilateral extracerebral space and cortical sulci obliteration. frontal extracerebral space is the most frequent site of extracerebral fluid collection. Bilateral symmetricity is promoinent in ty e I, trauma related intracranial lesions are frequently associated with type II and type III, All cases of Type Iis limited below two years of age. Bilateral cortical sulci widening is the finding of subarachnoid fluid collection and cortical sulci boliteration is strongly suggestive finding of subdural fluiiiiid colection.