The purpose of this study was to determine the effects of age and leg length on the energy-expenditure predictions of five activity monitors. Participants (N=86, ages 8-40 years) performed three progressive bouts of treadmill activity ranging from 4 to 12 km/hr. Differences between measured energy expenditure (VO2) and activity-monitor-predicted energy expenditure were assessed across five leg length categories to determine the influence of leg length. Accelerometer counts or pedometer steps along with age, weight, and leg length accounted for 85-94% of measured energy expenditure. The addition of age and leg length as predictor variables explained a larger amount of variance in energy expenditure across all speeds. Differences in leg length and age might affect activity-monitor validity and, therefore, should be controlled for when estimating physical activity energy expenditure.