RATIONALE: Body composition changes throughout life may explain the inconsistent associations reported between body mass index (BMI) and lung function in children. OBJECTIVES: To assess the associations of body weight and composition trajectories from 7 to 15 years with lung function at 15 years and lung function growth between 8 and 15 years. METHODS: Sex-specific BMI, lean body mass index (LBMI) and fat mass index (FMI) trajectories were developed using Group-Based Trajectory Modeling on data collected at least twice between 7 and 15 years from 6,964 children (49% boys) in the UK Avon Longitudinal Study of Parents and Children birth cohort. Associations of these trajectories with post-bronchodilation lung function parameters at 15 years and with lung function growth rates from 8 to 15 years were assessed using multivariable linear regression models, stratified by sex, in a subgroup with lung function data (n=3,575). MEASUREMENTS AND MAIN RESULTS: For all body mass measures we identified parallel trajectories that increased with age. There was no consistent evidence of an association between the BMI trajectories and lung function measures. Higher LBMI trajectories were associated with higher levels and growth rates of FVC, FEV1, and FEF25-75 in both sexes (e.g. boys in the highest LBMI trajectory had on average a 0.62L [95%CI: 0.44; 0.79, p-trend<0.0001] higher FVC at 15 years than boys in the lowest trajectory). Increasing FMI trajectories were associated with lower levels and growth rates of FEV1 and FEF25-75 only in boys and lower levels of FEV1/FVC in both sexes. CONCLUSIONS: Higher lean body mass during childhood and adolescence is consistently associated with higher lung function at 15 years in both sexes, whereas higher fat mass is associated with lower levels of only some lung function parameters.