["The purpose of this study was twofold: 1) to evaluate the effectiveness of a school-based physical activity intervention, Action Schools! BC (AS! BC), for maintaining academic performance in a multiethnic group of elementary children, and 2) to determine whether boys and girls' academic performance changed similarly after participation in AS! BC.", 'This was a 16-month cluster randomized controlled trial. Ten schools were randomized to intervention (INT) or usual practice (UP). One INT school administered the wrong final test, and one UP school graded their own test, so both were excluded. Thus, eight schools (six INT, two UP) were included in the final analysis. Children (143 boys, 144 girls) in grades 4 and 5 were recruited for the study. We used the Canadian Achievement Test (CAT-3) to evaluate academic performance (TotScore). Weekly teacher activity logs determined amounts of physical activity delivered by teachers to students. Physical activity was determined with the Physical Activity Questionnaire for Children (PAQ-C). Independent t-tests compared descriptive variables between groups and between boys and girls. We used a mixed linear model to evaluate differences in TotScore at follow-up between groups and between girls and boys.', 'Physical activity delivered by teachers to children in INT schools was increased by 47 min x wk(-1) (139 +/- 62 vs 92 +/- 45, P < 0.001). Participants attending UP schools had significantly higher baseline TotScores than those attending INT schools. Despite this, there was no significant difference in TotScore between groups at follow-up and between boys and girls at baseline and follow-up.'] Does school-based physical activity compromise children 's academic performance?