This study considered the validation of the Computer Science Applications (CSA) activity monitor (model 7164) for predicting activity energy expenditure (EE). A group of 34 Vietnamese adolescents (aged 11 to 15) performed three 5-minute treadmill trials at 4.5, 6.6, and 8.8 km · h–1. Mean activity counts and heart rate (HR) were significantly changed with the three-speed trials (p > 0.05). An equation to predict EE (kcal · min–1) was developed from activity counts and body mass (BM) from the 24 random subjects in Vietnam and was validated on the remaining 10 subjects. This equation explained 72% of the variability in kcal · min–1 (adjusted R2 = 0.72, SEE = 0.91 kcal · min–1). Consistent with previous studies, the relatively high SEE indicates that the equation is more suited for groups of Vietnamese adolescents rather than individuals.