We determined whether activity energy expenditure (AEE, from doubly labeled water and indirect calorimetry) or physical activity [7-day physical activity recall (PAR)] was more related to adiposity and the validity of PAR estimated total energy expenditure (TEE_PAR) in prepubertal and pubertal boys (n = 14 and 15) and girls (n = 13 and 18). AEE, but not physical activity hours, was inversely related to fat mass (FM) after accounting for the fat-free mass, maturation, and age (partial r = 0.35, P  0.01). From forward stepwise regression, pubertal maturation, AEE, and gender predicted FM (r² = 0.36). Abdominal visceral fat and subcutaneous fat were not related to AEE or activity hours after partial correlation with FM, maturation, and age. When assuming one metabolic equivalent (MET) equals 1 kcal · kg body wt¹ · h¹, TEE_PAR underestimated TEE from doubly labeled water (TEE bias) by 555 kcal/day ± 2 SD limits of agreement of 913 kcal/day. The measured basal metabolic rate (BMR) was >1 kcal · kg body wt¹ · h¹ and remained so until 16 yr of age. TEE bias was reduced when setting 1 MET equal to the measured (bias = 60 ± 51 kcal/day) or predicted (bias = 53 ± 50 kcal/day) BMR but was not consistent for an individual child (± 2 SD limits of agreement of 784 and 764 kcal/day, respectively) or across all maturation groups. After BMR was corrected, TEE bias remained greatest in the prepubertal girls. In conclusion, in children and adolescents, FM is more strongly related to AEE than activity time, and AEE, pubertal maturation, and gender explain 36% of the variance in FM. PAR should not be used to determine TEE of individual children and adolescents in a research setting but may have utility in large population-based pediatric studies, if an appropriate MET value is used to convert physical activity data to TEE data.