Drawing on medical anthropology and science and technology studies, I present a case study of the diagnosis of attention deficit hyperactivity disorder (ADHD) in Portugal. In a country where ADHD is a relatively recent medical category, still undergoing validation, the diagnosis is not primarily bound to an evidence-making role, and its epidemiology remains largely unknown. Notwithstanding, the diagnosis has been carried out by child psychiatrists and developmental clinicians who describe it as the most prevalent disorder that affects school-aged children and adolescents. In this article, I examine the global data, local diagnostic protocols, and clinical practices that are adapted and selectively mobilized in the making of the diagnosis, in a context in which making ADHD evident is at stake. The findings show that what counts as ADHD and what it means in each setting varies, and that the diagnosis may be understood as a situated process.