There has been widespread concern about the apparent dramatic increase in the prevalence of autism in the United States over the last decade. It remains unclear whether this represents a true increase in the number of children with the disorder, better case finding, or changes in diagnostic criteria. Olmsted County, Minnesota, provides a unique population-based laboratory to explore this question. Barbaresi and colleagues found that the incidence of autism, as defined using specific research criteria, increased among young children after the introduction of broader diagnostic criteria, increased availability of services, and increased awareness of autism. The timing of this increase suggests that the change in prevalence is at least partly due to these external factors.