A nationwide survey of children diagnosed with autism spectrum disorders (ASD) was conducted with the aim of exploring outcomes in adolescence and adulthood. The purpose of this article is to evaluate the representativeness of the study population and to describe the study population. In total, 1,734 parents and 933 individuals with ASD returned the questionnaire, which included information from individuals with ASD and their parents. The study population was found to be representative of individuals diagnosed with ASD in childhood at Danish psychiatric hospitals. Compared to other ASD populations, lower rates of intellectual disability, psychiatric comorbidity, language difficulties, and epilepsy were found.