The purpose of this study was to determine the validity of the Caltrac accelerometer for estimating 24-h energy expenditure (EE) in children and adolescents. EE for 40 girls (13.0 +/- 1.8 yr) was assessed for 24 h via indirect calorimetry in whole-room calorimeters. EE and activity level were estimated concurrently by two Caltrac accelerometers placed on the subjects at each hip. Significant correlations (P < 0.001) resulted between Caltrac estimates and calorimeter values for 24-h total EE (TEE, r = 0.80), sedentary daily EE (SDEE, r = 0.84), and waking EE (WEE, r = 0.85). Nonetheless, the Caltrac significantly (P < 0.001) underestimated EE in all experimental conditions (TEE: -13.3 +/- 8.6%; SDEE: -6.8 +/- 7.3%; WEE: -30.4 +/- 8.5%). A significant multiple correlation between calorimeter values and a combination of Caltrac activity counts and body weight (R = 0.86, P < 0.001) suggested these variables could be useful for daily EE estimation. Additional analyses indicated that as EE increased, the absolute difference between Caltrac and calorimeter values also increased. The significant correlations between Caltrac and calorimeter values suggest the Caltrac may be useful for assessing daily caloric expenditure for groups of children.