To examine the effects of time of day and playtime on hyperactive behaviour in children with ADHD, 14 boys with ADHD and 14 control children were observed and videotaped during a whole school day. Analyses reveal minor differential effects of time of day and playtime on the hyperactive behaviour of children with ADHD compared to control children for noisiness and out of seat behaviour. Children with ADHD show a stronger increase in hyperactive behaviour after playtime in the afternoon. These differential effects are important in the light of the management of children with ADHD in the classroom and of observations that commonly take place for both diagnosis and research.