This study investigates what role diagnoses play in institutional decision-making in regards to provision of special educational support and in the referral of pupils from mainstream schooling to special needs schools or classes. During the past decades, many countries have witnessed a radical increase in the number of children receiving psychiatric diagnoses, and increasing demands for special educational support. This development raises the question of how these two observations relate, and if psychiatric diagnoses serve as a prerequisite for allocating support. This study uses an institutional ethnographic approach to analyse qualitative data from four Danish municipal referral committees, processing 13 children's cases. In these settings, representatives of school authorities and educational psychologists decide if a child should be offered special educational support. The results show that the committees do not use diagnoses as direct arguments for their decisions. However, the institutional setup of the referral system creates an expectation that children referred to the committees already are diagnosed, as the types of special educational support are organised according to diagnostic categories. When a child is identified as experiencing problems at school, this expectation might prompt schools, parents or educational psychologists to request that a psychiatric assessment is carried out.