The history of psychiatry is the history of therapeutic enthusiasm with all of the triumph and tragedy, hubris and humility that this brings. As a result, the emergence of any new diagnosis, such as attention-deficit hyperactivity disorder (ADHD), needs to be greeted with caution, rigour and scientific objectivity, as well as compassion, therapeutic engagement and optimism. Although there is now little doubt that ADHD is a valid, useful diagnostic concept, and progress has been made, there is still considerable work to be done to establish its incidence, prevalence and biological underpinnings, as well as optimal therapeutic strategies. As with all mental illnesses, it is likely that knowledge will develop over many decades and diagnostic criteria will be refined in parallel. In the absence of definitive biological understanding, there should be particular caution about over-exuberant diagnostic expansionism unless it is accompanied by compelling evidence of therapeutic benefit for those diagnosed.