This study examined the treatment sensitivity of the ADHD Questionnaire (ADHD-Q), which is a brief rating scale for measuring symptoms of inattention, hyperactivity, and impulsivity in children. Parent, teacher, and child self-report data of the ADHD-Q were obtained for 17 clinically referred children with ADHD on the three occasions: (1) during the regular intake assessment, (2) just before the start of the stimulant medication (i.e., methylphenidate) intervention, and (3) four weeks after the start of the medication intervention. Results showed that ADHD-Q scores remained fairly stable in the period prior to the intervention, but then showed a substantial decline after the stimulant medication had been administered. Clearly, this finding supports the treatment sensitivity of the ADHD-Q.