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To validate the Computer Science and Application's (CSA) activity monitor for assessment of the total amount of physical activity during two school-weeks in 9-yr-old children and to develop equations to predict total energy expenditure (TEE) and activity energy expenditure (AEE) from activity counts and anthropometric variables. A total of 26 children (15 boys and 11 girls, mean age 9.1+/-0.3 yr) were monitored for 14 consecutive days. TEE was simultaneously measured by the doubly labeled water method. Averaged activity counts (counts. min (-1)) were compared with data on: 1) TEE, 2) AEE= TEE minus basal metabolic rate (BMR; estimated from predictive equations), and 3) daily physical activity level (PAL= TEE/BMR). Physical activity determined by activity counts was significantly related to the data on energy expenditures: TEE (r= 0.39; P< 0.05), AEE (r= 0.54; P< 0.01), and PAL (r= 0.58; P< 0.01). Multiple stepwise regression analysis showed that TEE was significantly influenced by gender, body composition (body weight or fat free mass), and activity counts (R (2)= 0.54--0.60). AEE was significantly influenced by activity counts and gender (R (2)= 0.45). There were no significant differences between activity counts and PAL in discriminating among activity levels with" low"(PAL< 1.56)," moderate"(1.57< or= PAL> or= 1.81), and" high"(PAL> 1.81) intensity. Activity counts from the CSA activity monitor seems to be a useful measure of the total amount of physical activity in 9-yr-old children. Activity counts contributed significantly to the explained variation in TEE and was the best predictor of AEE.