The definition of diagnostic categories, such as autism, is not always consensual. It can be the cause of political struggles between various actors, including professionals, public administrations or patient associations. However, little is known about the situation of patient or parent associations in these "diagnostic politics." We assert here that these associations are more sensitive to the politics of definition than is suggested by the current historiography. Through an analysis of discourses and strategies of the Quebec Autism Society from 1982 to 2017, we document the role that this association intended to play in the politics of autism and we show how the adoption by the state of diagnostic-based policies intensify definitional debates in civil society, including among parents.