OBJECTIVES: Examine longitudinal associations between modes of travel to school and adiposity. SETTING: The UK. PARTICIPANTS: 8432 children surveyed at ages 7, 11 and 14 years from the UK Millennium Cohort Study. PRIMARY AND SECONDARY OUTCOMES: Objective percentage body fat and body mass index (BMI). Transport mode was categorised as private motorised transport, public transport and active transport (walking or cycling). Socioeconomic position (SEP) was measured by household income group and occupational social class. We adjusted analyses for changes in the country of UK, frequency of eating breakfast, self-reported growth spurts, hours of screen time and days per week of moderate-to-vigorous physical activity. Longitudinal (panel) regression models adjusting for individual fixed effects examined associations in changes in mode of travel to school and adiposity, controlling for both time-varying and time-invariant potential confounders. Interaction tests and stratified analyses investigated differences by markers of SEP. RESULTS: At age 14 years, 26.1% of children (2198) reported using private motorised transport, 35.3% (2979) used public transport and 38.6% (3255) used active transport to get to school. 36.6% (3083) of children changed mode two times between the three waves and 50.7% (4279) changed once. Compared with continuing to use private transport, switching to active transport was associated with a lower BMI (-0.21 kg/m2, 95% CI -0.31 to -0.10) and body fat (-0.55%, 95% CI -0.80% to -0.31%). Switching to public transport was associated with lower percentage body fat (-0.43%, 95% CI -0.75% to -0.12%), but associations with BMI did not reach statistical significance (-0.13 kg/m2, 95% CI -0.26 to 0.01). Interaction tests showed a trend for these effects to be stronger in more deprived groups, but these interactions did not reach statistical significance. CONCLUSION: This longitudinal study during a key life course period found switching to physically active forms of travel can have beneficial adiposity impacts; these associations may be more apparent for more disadvantaged children. Increasing active travel has potential to ameliorate inequalities.