Background: Children with intellectual and developmental disabilities are at heightened risk for mental disorders, and disruptive behaviour disorders appear to be the most prevalent. The current study is a longitudinal examination of attention-deficit/hyperactivity disorder (ADHD) among children with and without intellectual disability (ID) across ages 5 to 8. Method: We assessed 228 5-year-old children, 87 with ID and 141 with typical development (TD), for clinical diagnoses using a structured interview. These interviews were conducted with mothers annually from child age 5 to 8. Results: Attention-deficit/hyperactivity disorder was over 3 times as prevalent in the ID group as in the TD group across ages 5, 6, 7 and 8. The diagnosis of ADHD tended to emerge earlier and was more stable in the ID group; however, the total number and relative frequency of ADHD symptoms endorsed appeared to be similar within the two groups across time. With respect to the developmental course, the trajectories of ADHD inattentive and hyperactive/impulsive symptoms over time were similar in the two groups. Discussion: Children with ID appear to be at heightened risk for ADHD and they may experience a longer and more persistent course of the disorder. These findings highlight the need for making interventions available for early treatment of this condition in children with ID.