The commentary by Arnold (2013) raises a number of interesting issues and speculations about the action of methylphenidate in children with intellectual disability (ID) and associated neurodevelopmental disorders, such as autism spectrum disorders. In our article (Simonoff et al., 2013), we were careful to stick closely to the statistical analysis plan drawn up (and approved by the Data Monitoring Committee) during data collection and prior to any exploratory analysis. However, a number of the issues raised by Arnold warrant further response, with the aim of clarifying the lessons that can reasonably be drawn for clinical practice.The authors of this article present their responses in this article. They conclude that the clinical implications suggested by Arnold (2013) provide a useful guide to treating children with ADHD/hyperkinetic disorder and intellectual and developmental disorders. (Contains 1 table.)