BACKGROUND: Wrist-worn accelerometers may provide an alternative to hip-worn monitors for assessing physical activity as they are easier to wear and may thus facilitate long-term recordings. The present study aimed at a) assessing the validity of the Actiwatch (wrist-worn) for estimating energy expenditure b) determine cut-off values for light, moderate and vigorous activities, c) studying the comparability between the Actiwatch and the Actigraph (hip-worn) and d) assessing reliability. METHODS: For validity, indirect calorimetry was used as criterion measure. ROC-analyses were applied to identify cut-off values. Comparability was tested by simultaneously wearing of the two accelerometers during free-living condition. Reliability was tested in a mechanical shaker. RESULTS: All-over correlation between accelerometer output and energy expenditure were found to be 0.80 (P<0.001).Based on ROC-analysis, cut-off values for 1.5, 3 and 6 METs were found to be 80, 262 and 406 counts per 15 s, respectively. Energy expenditure estimates differed between the Actiwatch and the Actigraph (p<0.05). The intra- and inter-instrument coefficient of variation of the Actiwatch ranged between 0.72%, and 8.4%. CONCLUSION: The wrist-worn Actiwatch appears to be valid and reliable for estimating energy expenditure and physical activity intensity in children aged 8 to 10 years.