evidence: Several reports of encephalopathy following measles vaccination can be found in the Vaccine Adverse Event Reporting System (VAERS) (submitted between November 1990 and July 1992). As with many VAERS reports, the information that is supplied is frequently inadequate to support or reject a diagnosis or to exclude the possibility that other factors are responsible for the disorder, if the case was encephalopathy or encephalitis. The committee found that 17 VAERS reports were suggestive of encephalopathy or encephalitis in vaccinees (mostly MMR) from ages 5 months to 16 years. Reported latencies ranged from 1 to 14 days after immunization. The patients presented with symptoms such as fever, ataxia, somnolence, convulsions, and flaccid paralysis. Several reports contained too little information to suggest a diagnosis or to shed light on causality. [...] There is demonstrated biologic plausibility that measles vaccine might cause encephalopathy. Although there are a number of reports of encephalitis or encephalopathy following immunization with measles vaccines of various strains, the rates quoted are impossible to distinguish from background rates. Good case-control or controlled cohort studies of these conditions in similar unvaccinated populations, which are necessary for determining the causal relation between measles and mumps and encephalopathy and encephalitis, are lacking. No conclusive evidence of the occurrence of encephalopathy or encephalitis resulting from the administration of measles vaccine was identified. There are no data regarding the occurrence of encephalopathy following administration of monovalent mumps vaccine. It is therefore not possible to implicate specifically either the measles or mumps component of MMR.