BACKGROUND It has been suggested that vaccination against measles, mumps, and rubella (MMR) is a cause of autism. METHODS In 2005, we conducted a cohort study of all children born in Denmark from January 1991 through December 1998. The cohort was selected on the basis of data from the Danish Civil Registration System, which assigns a unique identification number to every live-born infant and new resident in Denmark. MMR-vaccination status was obtained from the Danish National Board of Health. Information on these same children's autism status was obtained from the Danish Psychiatric Central Register, which contains information on all diagnoses received by patients in psychiatric hospitals and outpatient clinics in Denmark. We obtained information on potential confounders from the Danish Medical Birth Registry, the National Hospital Registry, and Statistics Denmark. RESULTS Of the 537,303 children in the cohort (representing 2,129,864 person-years), 440,655 (82.0 percent) had received the MMR vaccine. We identified 316 children with a diagnosis of autistic disorder and 422 with a diagnosis of other autistic-spectrum disorders. After adjustment for potential confounders, the relative risk of autistic disorder in the group of vaccinated children, as compared with the unvaccinated group, was 0.92, and the relative risk of another autistic- spectrum disorder was 0.83. A. What kind of cohort study is this, prospective or retrospective? B. What are two benefits of this type of cohort study versus the other? (Would give full credit if answers make sense. They don't need to be specifically these two.) C. Were children already classified as developing the outcome (Autism) prior to investigators starting this study? D. Were children already exposed (vaccinated) prior to investigators starting this study? E. Do the study results provide evidence that vaccination was a risk factor for autism?