BACKGROUND/OBJECTIVES: Easy-to-use and accurate methods to assess free-living activity energy expenditure (AEE) in preschool children are required. The aims of this study in healthy preschool children were to (a) evaluate the ability of the wrist-worn ActiGraph wGT3x-BT to predict free-living AEE and (b) assess wear compliance using a 7-day, 24-h protocol. SUBJECTS/METHODS: Participants were 40 Swedish children (5.5 +/- 0.2 years) in the Mobile-based intervention intended to stop obesity in preschoolers (MINISTOP) obesity prevention trial. Total energy expenditure (TEE) was assessed using the doubly labeled water method during 14 days. AEE was calculated as (TEEx0.9) minus predicted basal metabolic rate. The ActiGraph accelerometer was worn on the wrist for 7 days and outputs used were mean of the daily and awake filtered vector magnitude (mean VM total and mean VM waking). RESULTS: The ActiGraph was worn for 7 (n = 34, 85%), 6 (n = 4, 10%), 5 (n = 1, 2.5%) and 4 (n = 1, 2.5%) days (a valid day was. 600 awake minutes). Alone, mean VM total and mean VM waking were able to explain 14% (P = 0.009) and 24% (P = 0.001) of the variation in AEE, respectively. By incorporating fat and fat-free mass in the models 58% (mean VM total) and 62% (mean VM waking) in the variation of AEE was explained (P amp;lt; 0.001). CONCLUSIONS: The wrist-worn ActiGraph wGT3x-BT in combination with body composition variables explained up to the 62% of the variation in AEE. Given the high wear compliance, the wrist-worn ActiGraph has the potential to provide useful information in studies where physical activity in preschool children is measured.