Reye syndrome (RS) is an acute metabolic encephalopathy and hepatopathy affecting children and adolescents. Outbreaks of RS were common in United States until the early 1980s. However, after the abolition of salicylate (aspirin) therapy for infectious diseases such as influenza or varicella in patients under 18 years of age the incidence decreased. Now classical RS is rare and RS is considered a secondary mitochondrial disease. Reye-like syndrome (RLS), resulting from congenital errors of mitochondrial fatty oxidation, especially medium-chain acyl-CoA dehydrogenase deficiency, has increased due to progress in diagnostic techniques and methods after 1990. Diagnostic differentiation between RS and RLS is difficult because the end results of mitochondrial dysfunction in RS and RLS may be similar.