A total of 3147 pupils from Belgium, Germany, Greece, Hungary and Norway participated in a school-randomized controlled trial. The intervention included 1–2 school lessons per week for a period of six weeks, along with assignments for the children and their parents. Screen time and breaking up sitting time were registered by self-report and total sedentary time and breaking up sitting time by accelerometry. The effect of the intervention on these behaviors was analysed by multilevel regression analyses. All analyses were adjusted for baseline values and gender. No significant intervention effects were observed, neither for self-reported TV/DVD or computer/game console time, nor for accelerometer-assessed total sedentary time and number of breaks in sitting time. The intervention group did report more positive attitudes towards and preferences/liking for breaking up sitting time than the control group. Overall, these results do not warrant wider dissemination of the present UP4FUN intervention.