The relationship between intracranial pressure (ICP) and outcome was studied in 10 adults with encephalitis. Eight had biopsy-proven herpes simplex encephalitis, one had acute hemorrhagic leukoencephalitis, and in one case the cause of encephalitis was unknown. Monitoring of ICP was instituted because of clinical deterioration or computerized tomography evidence of brain swelling, and was begun a mean of 7 days after the onset of symptoms and continued for a mean of 9 days. All five survivors, but only one of the five fatalities, had an initial ICP of less than 12 mm Hg (p less than 0.05). Four patients with a mean daily ICP of less than 20 mm Hg survived, whereas five of six patients with higher ICP's died (p less than 0.05). Peak ICP did not occur until the 12th day of illness on average. The Glasgow Coma Scale score at the time the ICP monitor was inserted did not correlate with outcome. Intracranial pressure monitoring in severe encephalitis may be a useful adjunct for therapy and an indicator of prognosis.