Hepatic encephalopathy and brain edema are important complications in the course of a patient with acute liver failure. Presumed unrelated for many years, increasing evidence suggests that an increase in brain water is seen in all forms of hepatic encephalopathy. Ammonia, traditionally linked to the pathogenesis of hepatic encephalopathy, plays an important role in the increase in brain water. In acute liver failure, an osmotic disturbance in the astrocyte, in combination with an alteration of cerebral blood flow results in overt brain edema and intracranial hypertension. In cirrhosis, magnetic resonance techniques indicate the presence of a brain osmotic disturbance. Several clinical factors modulate the development of brain swelling.