This paper examines the change in body mass index for a cohort of Irish children as they aged from 9 to 13 and decomposes the change into parts attributable to changes in observable characteristics and changes in returns to observable characteristics. The decomposition is carried out over the whole of the distribution, with a particular focus on the upper percentiles and a number of different decomposition techniques are applied and compared. The overall increase in BMI at higher percentiles is modest and is over-explained by the change in characteristics and is not sensitive to the adopted technique. The paper also carries out a number of partial equilibrium counterfactuals examining the impact of non-marginal changes in variables such as exercise and maternal education. The impact of these counterfactuals is limited.