The objective of this study is to compare the time trend of reported diagnoses of autism spectrum disorder (ASD), hyperkinetic disorder, Tourette’s syndrome, and obsessive-compulsive disorder across four countries after standardizing the study period, diagnostic codes used to define the conditions and statistical analyses across countries. We use a population-based cohort, including all live-born children in Denmark, Finland, Sweden and Western Australia, from January 1, 1990, through December 31, 2007 and followed through December 31, 2011. The main outcome measure is age-specific prevalence of diagnoses reported to population-based registry systems in each country. We observe an increase in age-specific prevalence for reported diagnoses of all four disorders across birth-year cohorts in Denmark, Finland, Sweden, and (for ASD) Western Australia. Our results highlight the increase in the last 20 years in the number of children and families in contact with health care systems for diagnosis and services for an array of childhood neuropsychiatric disorders, a phenomenon not limited to ASD. Also, the age of diagnosis of the studied disorders was often much higher than what is known of the typical age of onset of symptoms, and we observe limited leveling off in the incidence rate with increasing age.