The prevalence of Attention-Deficit/Hyperactivity Disorder (ADHD) diagnoses and medication use has increased over time around the world, but significant regional differences remain. This paper aims to determine and explain disparities in ADHD prevalence and medication use among school-aged children in two distinct school systems, in Flanders (Belgium) and Québec (Canada). We present detailed descriptive and comparative analyses of data from 35 schools, 114 teachers, and 1046 parents (children) that were collected as part of a comparative international project. The data concern teacher and parent suspicions, teachers’ ratings of ADHD-related behaviors in children, teachers’ views of medication use, and teachers’ beliefs about ADHD. The results show that, compared with Flanders, Québec had significantly more children diagnosed with ADHD and more frequent suspicions of ADHD in children by teachers and parents. We refer to the conceptual, institutional, and interactional levels of medicalization to interpret our findings and conclude that social and cultural readings of children’s behaviors differ greatly between regions. Medicalization of children’s behaviors is more common in Québec than in Flanders.