Reye syndrome has emerged as the quintessential example of an acute metabolic encephalopathy. The clinical presentation is quite stereotyped in most instances permitting rapid, accurate diagnosis and early therapeutic intervention. Intoxications and certain inborn metabolic errors may mimic Reye syndrome. All patients with a recurrent episode should be investigated thoroughly for evidence of a metabolic disorder associated with an enzyme deficiency. Notable in this regard are inborn errors affecting organic acid, ammonia, and carbohydrate metabolism. The mitochondrial disturbance in Reye syndrome is well documented but the pathophysiologic sequence linking the antecedent viral illness to the mitochondrial injury remains obscure. Recent identification of a spontaneous Reye-like illness in mice that is associated with a coronavirus infection may provide an opportunity to investigate this initial phase of the pathophysiologic sequence. The primary cerebral insult presumably derives from insufficient substrate availability and results in massive cytotoxic cerebral edema. Treatment revolves around the continuous infusion of hypertonic glucose and intermittent infusion of hypertonic mannitol. Management is designed to attenuate or avoid the various compounding metabolic insults during this critical period when the child is metabolically crippled. In 1963, the disorder was considered to be rare and almost irreversibly fatal. Today, the disorder is recognized to be more common, and the outcome is very satisfactory in 85 to 90 per cent of the cases. The role of aspirin remains very controversial. A number of studies suggest an association between this potential mitochondrial toxin and Reye syndrome, but a causal relationship has not been established. Until better understood, it seems advisable to avoid use of aspirin in children exhibiting symptoms suggestive of Reye syndrome.