Objective. To examine individual changes in energy expenditure and physical activity during prepubertal growth in boys and girls. Methods. Total energy expenditure (TEE), resting energy expenditure, physical activity- related energy expenditure, reported physical activity, and fat and fat-free mass were measured three times over 5 years in 11 boys (5.3 ± 0.9 years at baseline) and 11 girls (5.5 ± 0.9 years at baseline). Results. Four-year increases in fat (~6 kg) and fat-free mass (~10 kg) and resting energy expenditure (~200 kcal/day) were similar in boys and girls. In boys, TEE increased at each measurement year, whereas in girls, there was an initial increase from age 5.5 (1365 ± 330 kcal/day) to age 6.5 (1815 ± 392 kcal/day); however, by age 9.5, TEE was reduced significantly (1608 ± 284 kcal/ day) with no change in energy intake. The gender difference in TEE changes over time was explained by a 50% reduction in physical activity (kcal/day and hours/week) in girls between the ages of 6.5 and 9.5. Conclusions. These data suggest a gender dimorphism in the developmental changes in energy expenditure before adolescence, with a conservation of energy use in girls achieved through a marked reduction in physical activity.