Although evidence suggests that Attention Deficit Hyperactivity Disorder (ADHD) results in higher health care and social costs, the design and sampling procedures of most existing studies are limited in various respects. A majority of studies have been conducted using American health insurance databases, where belonging to a specific health insurer is indicative of socio-economic status. The aim of the present study paper is to address the many methodological limitations in the current health economics literature on ADHD by tapping into the unique Central Person Register (CPR) databases in Denmark. (Series: The Rockwool Foundation Research Unit - Study Paper - No. 70).