In the past three decades, the number of overweight children aged 6-12 years has tripled and the number of overweight adolescents aged 12-19 years has doubled. The underlying reason for the obesity epidemic is complex, but reduced physical activity is likely a major contributor. Therefore, while quantifying energy expenditure is important, relatively little research has been completed with preschool children. Many studies have validated accelerometers in adults and adolescents, but few have validated them with preschool children. Therefore, the purpose of this study was to validate the Actiheart accelerometer in a preschool population while they performed age appropriate physical activities. Seventeen healthy preschoolers, aged 4-6 years, were recruited from the general public in Delaware. For validation, subjects walked at 2, 3, and approximately 4 mph (or maximal walking speed). Subjects wore the VmaxST, Actiheart accelerometer, and Actical accelerometer during data collection. This study compared activity energy expenditure (kcals/min) measured by the Actiheart to O2 (kcals/min) measured by the gold standard, indirect calorimetry (VmaxST). The Actical was only used for marking the Actiheart data. For the validation analysis, the mean and standard deviation for energy expenditure was calculated for each child and then the entire group. A 2 x 4 ANOVA was used to PREVIEW x determine if any significant differences existed between activity energy expenditure (kcals/min) measured by the VmaxST and the Actiheart monitor for each level of workload (rest, 2, 3, 4 mph/maximal walking speed). Regression analysis explored the ability of the Actiheart to predict O2 at each workload (rest, 2, 3, 4 mph). Activity energy expenditure (AEE [AC + HR]) from the Actiheart was entered into the regression analysis. Correlation coefficients were calculated to determine the strength of the relationship. Significance was set at p ≤ 0.05. There was no significant difference between AEE (kcals/min) measured by the VmaxST and Actiheart for each level of workload [F (2, 32) = 2.4, p = 0.13]. The correlation between the VmaxST and Actiheart was r= 0.71. The calibration equation established was O2 (kcals/min) = 0.37 + [0.75* Actiheart (kcals/min)], (r2 = 0.51, F (1, 49) =51.0, p < 0.01). In conclusion, the Actiheart is a valid instrument for measuring AEE (kcals/min) and predicting was O2 (kcals/min) in preschool children.