Objective The aim of this study was to identify if there is an association between physical activity, body mass and academic attainment in primary school children. Methods Eighty-six children at a UK primary school were included in this cohort analysis. Physical activity status was determined using the Physical Activity Questionnaire – Children. Weight and height was measured, and BMI calculated at 4-time points. Academic attainment was measured from national standardised tests. Results Children who are less active demonstrated lower height (mean difference (MD) 0.49 95% CI 0.08 to 0.90), weight (MD 0.58 95% CI 0.12 to 1.04) and BMI z-scores (MD 0.48 95% CI -0.04 to 1.00) than children who are more active. They also had a higher rate of weight gain (0.06 z-score units/month), than children who are more active (0.05*z-core units/ month), and had greater fluctuations in weight. Children who were more active performed significantly better than children who are less active in writing (?2 16.40, p=0.003) and mathematics (?2 12.18, p=0.02). Conclusion There does appear to be an association between physical activity, body mass and academic attainment in primary school children, such that lower activity levels negatively effects growth and academic performance. These differences could not be solely explained by physical activity level due to unaccounted socio-economic factor.