Objectives: To examine differences in sedentary time and bouts during and outside of childcare/school periods, and changes in sedentary time and bouts over 1-year among children who remained in childcare (childcare subsample) and among those who transitioned to school (school transition subsample). Design: Longitudinal study. Methods: Results are based on 177 children aged 3-5 years at baseline from the Healthy Active Preschool and Primary Years study in Melbourne, Australia. Sedentary time and sedentary bouts (1-4, 5-9, [greater than or equal to]10 min) for total days and during/outside of childcare/school on weekdays were accelerometer-derived at baseline (2008) and 1-year follow-up (2009), when 57% of participants had transitioned to school. Repeated-measures ANCOVAs adjusting for wear time were conducted. Results: Compared to the outside of childcare/school period, children in the school transition subsample spent more time (0.5 min/day or 0.9% wear time) in [greater than or equal to]10 min sedentary bouts at baseline, participated in 26 more min/day of sedentary time at follow-up, and all participants spent less time (2-16 min/day or 2-3% of wear time) in 5-9 min sedentary bouts at baseline and follow-up during the childcare/school period (P 0.05). Increases in sedentary time (34-54 min/day or 2-3% wear time) and time spent in 1-4 min sedentary bouts (18-29 min/day or 1-2% of wear time) were observed from baseline to follow-up in both the total sample and school transition sub-sample, for total days and during the childcare/school period (P 0.05). Conclusions: School transition was marked by increased sedentary time. School practices, policies, and environments to reduce sedentary time should be explored. (author abstract)