This study developed algorithms for predicting activity energy expenditure (AEE) in children (n = 24) and adults (n = 24) from the Actical[R] activity monitor. Each participant performed 10 activities (supine resting, three sitting, three house cleaning, and three locomotion) while wearing monitors on the ankle, hip, and wrist; AEE was computed from oxygen consumption. Regression analysis, used to create AEE prediction equations based on Actical[R] output, varied considerably for both children (R[squared] = 0.45-0.75; p less than 0.001) and adults (R[squared] = 0.14-0.85; p less than 0.008). Most of the resulting algorithms accurately predicted accumulated AEE and time within light, moderate, and vigorous intensity categories (p greater than 0.05). The Actical[R] monitor may be useful for predicting AEE and time variables at the ankle, hip, or wrist locations. (Contains 7 tables and 6 figures.)