Raised intracranial pressure due to cerebral oedema is a major cause of death in fulminant hepatic failure and in the present study we have carried out a controlled clinical trial of continuous hyperventilation in the prevention of this complication. Twenty patients were electively hyperventilated to maintain PaCO2 between 3.5 and 5 kPa. In the other 35 patients mechanical ventilation was instituted only if severe hypoxia or hypercapnia occurred. Cerebral oedema, diagnosed clinically or by a rise in intracranial pressure to greater than 30 mm Hg, occurred in 85% of hyperventilated patients and in 86% of those not so treated. Although there was no significant reduction in the number of episodes of cerebral oedema in the hyperventilated patients (4.8 episodes/24 h) as compared with the controls (5.3 episodes/24 h), hyperventilation did appear to delay the onset of coning but on the basis of these results could not be recommended to be used routinely as a prophylactic measure in the prevention of cerebral oedema in this condition.