The article presents an analysis of how diversity is defined and attended to in Swedish schools. The research reported has been carried out as a case study of categorizing practices that concern the uses of neuropsychiatric diagnoses, notably ADHD. The data were collected as part of a larger study. The interaction over a two-year period between the parents of a boy (William, aged 5.5 years) and representatives of the school (school psychologist, principal, teacher etc.) has been analysed. It is shown that parents and professionals provide different accounts of William's difficulties. The parents, while not denying that their son causes problems in class, argue that the boy will mature and that the problems will disappear. The representatives of the school try to convince the parents that a neuropsychiatric examination of the boy will be beneficial to all parties. The long process of negotiation can be understood as a rhetorical drama, where the category "ADHD" serves as the resolution of a complex institutional problem in the modern welfare state.