The Computer Science Application (CSA) accelerometer is designed to detect the acceleration and deceleration of human movement. The purpose of this study was to evaluate its validity and utility for measuring children's activity in field settings. The criterion measure was heart rate telemetry. This study also examined the between-day stability of children's physical activity. Thirty-one children (7-15 yr) wore a CSA accelerometer and a heart rate telemetry monitor for 12 h.d-1 for 3 consecutive days. The validity correlation coefficients between accelerometry and heart rate telemetry for each monitored day ranged from r = 0.50-0.74. All children reported the accelerometer to be comfortable to wear. The moderate to high validity correlations and the subjects' favorable response to wearing the accelerometer support its validity and utility as an objective method for monitoring activity in children in field settings. However, the between-day stability of individual physical activity measures was low to moderate (r = -0.23 to 0.53), indicating that when using accelerometry or heart rate telemetry more than 3 d of monitoring is needed to assess usual activity.