Minute-by-minute heart rate monitoring and an activity diary were used simultaneously during three days in 30 randomly selected adolescents (16 boys, 14 girls; mean age 15.0+/-1.0). Total daily energy expenditure (TDEE) and its components (energy expenditure during sleep, during rest and in physical activity) and times spent at different intensity levels (sedentary, light, moderate physical activity and vigorous physical activity) were compared. TDEE from heart rate monitoring averaged 10.9+/-2.7 MJ x d(-1) compared to 11.3+/-2.3 MJ x d(-1) from the activity diary (NS). The limits of agreement (mean+/-2 SD) were -3.54 MJ x d(-1) and 2.74 MJ x d(-1). There was no significant difference for any of the TDEE components between the methods (MANOVA). A significant method effect (P<0.001) was observed for time spent in sedentary and light physical activity (MANOVA). No significant difference was observed for time spent in moderate and vigorous physical activity. According to this, heart rate monitoring and activity diary are comparable for group assessment of TDEE and its components, and for estimating time spent in moderate and vigorous physical activity. The activity diary underestimated time spent in moderate and vigorous physical activity for inactive subjects and consequently overestimated highly active subjects.