A major challenge to studying autism with a suspected vaccine-related regression is identifying children with acute regressive-type symptoms following MMR vaccination; there are no specific codes, tests, or procedures that identify this occurrence with a high degree of specificity. This study will explore the Kaiser Permanente electronic databases to ascertain whether we can identify children with regressive type autism, and identify the timing of the regression in relation to the period directly following MMR vaccination. In order to see if identification of regressive autism from medical records is possible, the investigators will attempt to identify children vaccinated with MMR who then abruptly undergo a ‘cluster’ of visits, tests, and/or procedures in the time period directly following vaccination. The researchers feel that there may be a number of children who receive a diagnosis (such as ‘prolonged crying’) in the emergency department on the day after vaccination, followed shortly thereafter (1-2 days later) by another set of diagnoses (such as ‘fever’, & ‘irritability’) in the pediatric office or other outpatient department, and then receive either diagnostic or laboratory tests indicating (at least) a moderate degree of severity of concern, such as CT scans, metabolic testing or referral to neurology. If this study is successful in using medical databases to identify a specific group of children with demonstrable autism-related regression that clearly follows vaccination, it may point to the feasibility of further studies concentrating on this specific population.