--- - |2+ Abstract Background Although attention deficit hyperactivity disorder (ADHD) is considered a valid diagnosis for children with intellectual disability, no studies have evaluated the invariance of ADHD symptom ratings across children with and without intellectual disability. Method Parents completed ratings on the ADHD symptoms for 189 children with intellectual disability and for 474 children without intellectual disability. Differential item functioning analysis was used to determine the equivalence of the ADHD symptoms across the two groups. Results The symptoms loses things, talks too much, and blurts out answers showed significant bias against children with intellectual disability. The prevalence of ADHD in children with intellectual disability was 18% (according to the symptom criterion), and 7.4% when the academic and/or social impairment criterion was also considered. Conclusions Most of the ADHD symptoms can be valid for the assessment of ADHD in children with mild and moderate intellectual disability. ADHD symptoms may be used in further studies to establish base rates of the disorder in the intellectual disability population. - Journal of Applied Research in Intellectual Disabilities, EarlyView.