Existing research suggests that children with intellectual disabilities are at increased risk for ADHD, and that the symptoms of the disorder might successfully be treated with stimulant drugs. However, there has been little exploration of ADHD symptoms and their correlates in children with intellectual disabilities. Analyses of three samples of children with intellectual disabilities are presented (total N=338). Correlational analyses showed that younger children, and those with a diagnosis of Autism were rated as having more ADHD/hyperactivity symptoms. There was little evidence of a sex difference, and no strong associations with domains of adaptive behavior (socialization, communication, and daily living skills). However, there was a small but significant negative association between mental age and ratings of symptoms. Finally, an increased prevalence of ADHD/hyperactivity symptoms was confirmed in the children with intellectual disabilities compared to their siblings. This effect remained after controlling for chronological and mental age differences between the siblings. These findings support those from previous research and suggest that ADHD/Hyperkinesis may be a valid psychiatric diagnosis for children with intellectual disabilities. However, a great deal more research is needed to explore the phenomenology of ADHD in intellectual disability and to develop an evidence base for psychosocial intervention.