Reye syndrome is an acute metabolic encephalopathy, largely affecting children and adolescents. In Reye-like syndrome, because of inborn errors of metabolism, hypoglycemia, hypoketonemia, elevated ammonia, and organic aciduria are often evident. It is well-known that fatty-acid oxidation defects can present as Reye-like syndrome. The most commonly diagnosed metabolic disorder in association with Reye syndrome has been medium-chain acyl coenzyme A dehydrogenase deficiency. The present consensus seems to be that Reye syndrome is very rare, and that any child suspected of manifesting this disorder should undergo investigations for inborn errors of metabolism. We recently treated a child with "Reye-like illness" who possibly manifested a long-chain acyl dehydrogenase deficiency, and who had also ingested aspirin. We discuss the possible pathogenesis of the disorder in this child. The end results of mitochondrial dysfunction in Reye syndrome and Reye-like illness may be similar.