AIM To investigate whether increased physical exercise during the school day influenced subsequent cognitive performance in the classroom.METHOD A randomized, crossover-design trial (two weeks in duration) was conducted in six mainstream primary schools (1224 children aged 8-11y). No data on sex was available. Children received a teacher-directed, classroom-based programme of physical exercise, delivered approximately 30 minutes after lunch for 15 minutes during one week and no exercise programme during the other (order counterbalanced across participants). At the end of each school day, they completed one of five psychometric tests (paced serial addition, size ordering, listening span, digit-span backwards, and digit-symbol encoding), so that each test was delivered once after exercise and once after no exercise.RESULTS General linear modelling analysis demonstrated a significant interaction between intervention and counterbalance group (p<0.001), showing that exercise benefitted cognitive performance. Post-hoc analysis revealed that benefits occurred in participants who received the exercise intervention in the second but not the first week of the experiment and were also moderated by type of test and age group.INTERPRETATION Physical exercise benefits cognitive performance within the classroom. The degree of benefit depends on the context of testing and participants' characteristics. This has implications for the role that is attributed to physical exercise within the school curriculum.There have long been anecdotal reports of neuropsychological benefits arising from regular physical exercise, and these now receive support from a substantial body of empirical evidence. Recent reviews suggest that physical exercise can be useful in the treatment of depression, 1 can be used to maintain cognitive vitality in old age, 2 and can even reduce stereotypic and disruptive behaviours in children with autism, learning disorders, or behavioural disorders. 3 In children, associations between greater aerobic fitness and higher cognitive functioning have been reported. 4 Also, there is evidence of aerobic fitness level moderating aspects of academic performance 5 and children exhibiting increased 'ontask' classroom behaviour after brief exercise interventions. 6 Increases in the amount of curriculum time devoted to physical activity have also been reported to result in either no change or an improvement in students' academic achievement, 7 suggesting that increased exercise capably counteracts the negative impact of reduced academic instruction time, perhaps through improving cognition.Given these findings, recent initiatives to encourage increased childhood physical activity 8 may have important implications for mental as well as physical health. In adults executive functions such as working memory, goal-oriented processing, and inhibition are particularly improved by exercise interventions. 9,10 If this also applies in children, one might expect that the largest effects of exercise would be seen in individuals who have po...