Objective:To examine independent and combined cross-sectional associations between movement behaviors (physical activity [PA], sedentary time, sleep duration, screen time and sleep disturbance) and fat mass index (FMI) as well as to examine longitudinal associations between movement behaviors and FMI.Methods:Cross-sectional and longitudinal analyses were done using data from the OPUS school meal study on 785 children (52% boys, 13.4% overweight, ages 8-11). Total PA, moderate-to-vigorous physical activity (MVPA), sedentary time and sleep duration (seven days and eight nights) were assessed by accelerometer and FMI was determined by dual-energy X-ray absorptiometry (DXA) on three occasions over 200 days. Demographic characteristics, screen time and sleep disturbance (Children's Sleep Habits Questionnaire [CSHQ]) were also obtained.Results:Total PA, MVPA and sleep duration were negatively associated with FMI, while sedentary time and sleep disturbances were positively associated with FMI (P0.01). However, only total PA, MVPA and sleep duration were independently associated with FMI after adjustment for multiple covariates (P0.05), but higher FMI at baseline predicted a decrease in total PA and MVPA, and an increase in sedentary time (P0.001), even in normal-weight children (P0.03).Conclusion:Total PA, MVPA and sleep duration were independently associated with FMI, and combined associations of movement behaviors showed a synergistic effect with FMI. In the longitudinal study design, a high FMI at baseline was associated with lower PA and higher sedentary time after 200 days but not vice versa, even in normal-weight children. Our results suggest that adiposity is a better predictor of PA and sedentary behavior changes than the other way around.International Journal of Obesity accepted article preview online, 5 December 2013. doi:10.1038/ijo.2013.229.