evidence: The 4-year risk of autism increased nearly fourfold from 8 per 10,000 person-years in 1988 to 29 per 10,000 person-years in 1993, while the prevalence of MMR vaccination remained constant at 97%. The authors hypothesized that if MMR vaccine were a major cause of the increasing incidence of autism, then the risk of autism in successive birth cohorts would be expected to stop rising within a few years of the vaccine being widely used. However, because the incidence of autism among 2- to 5–year-olds increased markedly from 1988 to 1993 while MMR vaccine coverage was over 95% for successive birth cohorts, the authors conclude that the results do not support a causal association between MMR vaccination and the risk of autism.