The authors study 26 children with macrocranium and pericerebral effusion as diagnosed by C.T. Three groups are described: 10 with the classical C.T. features of subdural effusion, 12 with features of external hydrocephalus and 4 with massive pericerebral effusion. C.T. performed after lumbar subarachnoid injection of contrast medium, shows that all the effusions are subdural. In only one case, opacification of the subdural space occurred. The authors conclude that plain C.T. alone is unable to demonstrate the precise localization of pericerebral effusion in children and that all these effusions are, in their experience, subdural in localization.