In this journal, Holden, Jenkins-Jones, Poole, Morgan, Coghill and Currie , CAPMH 7:34, 2013, report on the prevalence and financial costs of treating people with attention deficit hyperactivity disorder (ADHD) in the UK over the last ten years. We commend the authors on their thorough cost analysis, and discuss differences in prevalence estimates of diagnosed ADHD, that is the proportion of the child population with an ADHD diagnosis, which varies dramatically between studies. We also discuss the reasons for this. Regional variation in application of diagnostic criteria and clinical subjectivity are likely partial explanations.