Aim: We have previously demonstrated improved cognitive performance after a classroom-based exercise regime. In this study, we examined the reproducibility of this effect in a more socio-economically diverse sample and also investigated whether cognitive benefits of exercise were moderated by body mass index (BMI) or symptoms of attention-deficit-hyperactivity disorder (ADHD). Method: A crossover design trial (2wks in duration) randomized 552 children (mean age 9y 8mo, SD 1y 2mo; range 8-12y) by their school into two counterbalanced groups. Children were eligible to participate provided that they did not receive any additional support. One group received a classroom-based programme of physical exercise on week 1 and then no programme on week 2, and this order was reversed for the other group. Each week, all participants completed a cognitive test battery that was delivered in one part per day at the end of each school day. Results: On the cognitive tests, a significant interaction between counterbalance group and exercise was observed (p less than 0.001). Benefits occurred only for participants who exercised during the second week (mean improvement mean 3.85, standard error 1.39). Although test scores were affected by age, sex, and level of ADHD symptoms, the effect of exercise was not moderated by either these factors or BMI. Interpretation: Exercise interventions have a positive effect (with variable magnitude) on cognitive performance, possibly by facilitating practice effects. These effects are not moderated by sex, ADHD symptom level, or BMI.