Objective: To determine the effect of a moderate intensity exercise programme on the behaviour of children with attention-deficit/hyperactivity disorder (ADHD). Design: Thirteen ADHD children (5-13 years old, 10 boys and 3 girls) participated in a 5-week exercise programme (5 days a week, 60 minutes of exercise, 20 minutes at 50-75% of maximal heart rate). Six ADHD children (5-13 years old, 3 boys, 3 girls) acted as non-exercising controls. A modified Conner's Parent Rating Scale was used to rate the children's behaviour 1 week before, after 3 weeks of exercise, and immediately after the 5-week period. Main outcome measures: The scale results (total behaviour, attentive behaviour, task orientation, emotional behaviour, motor skills, and oppositional behaviour) were statistically analysed using analysis of variance (ANOVA) with repeated measures. Level of significance was set at p < 0.05. Results: There were significant improvements in the behaviour of both the exercise and control groups immediately after cessation of the programme the following behaviour categories were significantly improved compared with 1 week before initiation of the programme: total behaviour (p = 0.01), attention (p = 0.008), emotional (p = 0.01), and motor skills (p = 0.004). Conclusion: Contrary to expectations the behaviour of all the ADHD children in the study improved over the 50week exercise programme. This suggests that it may have been the extra attention paid by parents, guardians and exercise leaders or interaction/co-operation with peers, which altered behaviour rather than the influence of the exercise programme.