The objective of this study was to obtain the basic data for developing an accurate and simple method to assess the daily physical activity of children. A portable expiration gas metabolic monitor was used to measure energy expenditure during several daily activities in fifteen children of 9 to 15 years of age. The triaxial accelerometer counts of the wrist, waist and ankle, and the heart rate were also measured. Energy expenditure during daily activities was strongly correlated with the increase in heart rate from that while sitting at rest in both boys and girls. The triaxial accelerometer counts of the wrist, waist and ankle were significantly related to energy expenditure; however, the correlation coefficient of the triaxial accelerometer counts of the wrist with energy expenditure was lower than that of the waist or ankle for both the boys and girls. A multiple regression analysis indicated that 84-88% of the variation in energy expenditure could be explained by the gender, body size, heart rate and triaxial accelerometer counts.