Introduction: It has been suggested that the measles-mumps-rubella (MMR) vaccination causes autism., Material and Methods: We conducted a retrospective cohort study of all children born in Denmark from January 1991 through December 1998. The cohort was established based on data from the Danish Civil Registration System. A unique person identifiable number given to all subjects enabled linkage with other national registries. MMR vaccination status was obtained from the Danish National Board of Health. Information on the children's autism status was obtained from the Danish Psychiatric Central Register which contains information on all diagnoses received from psychiatric hospitals, psychiatric wards, 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: In the cohort of 537,303 children (2,129,864 person-years), 440,655 children had been MMR vaccinated. We identified 316 children with a diagnosis of autistic disorder and 442 with a diagnosis of other spectrum disorders. After adjusting for potential confounders, the risk for autistic disorder and other spectrum disorders was not increased in vaccinated compared with unvaccinated children (relative risk 0.92; 95 percent confidence interval, 0.68 to 1.24 and relative risk 0.83; 95 percent confidence interval, 0.65 to 1.07). There was no association between age at vaccination, time since vaccination or calendar period at time of vaccination and development of autistic disorder., Discussion: This study provides strong evidence against the hypothesis that MMR vaccination causes autism.