To determine the importance of intracranial hypertension in central nervous system acute infections, we studied intracranial pressure (ICP) in 27 patients, aged 45 days to 13 years. Fourteen had meningitis and 13 had encephalitis; all were in deep coma with a Glasgow Coma Scale 7 or less. Intracranial hypertension defined by a mean ICP above 15 mm Hg was observed in 12 patients with meningitis (86%) and in 9 with encephalitis (69%). Patients with meningitis exhibited a very early and severe intracranial hypertension. A striking difference is noted between survivors and non-survivors who had a very high maximal ICP with a severe reduction in cerebral perfusion pressure. Intracranial hypertension occurred in all patients with acute primitive encephalitis but only in 3/7 patients with post-infectious encephalitis. ICP monitoring seems to be important in the comatose forms of bacterial meningitis in the early period, herpes encephalitis and postinfectious encephalitis with severe status epilepticus.