Eight men were fed at maintenance for 2 wk, followed by 4 wk at 50% of maintenance, then 1 wk at maintenance. The diets were formulated to contain either 40% or 20% of energy from fat. Daily energy expenditure (24-h EE) was determined by indirect calorimetry at the end of the 2-wk maintenance period; on days 1, 7, and 28 of reduced energy intake; and on days 1 and 7 of refeeding. During the reduced-energy period, body weight decreased from 96.6 to 91.5 kg and body fat decreased from 30.4% to 27.7%. There were no significant differences in 24-h EE or energy requirements per unit body weight due to diet composition or weight loss. Maintenance metabolizable-energy (ME) requirement averaged 31.0 kcal/kg body wt. Overall, the efficiency of ME use relative to body stores was 0.87 and was greater for high-fat than for low-fat diets. There was some evidence of an increase in the efficiency of energy use of body stores after weight loss. Substrate use reflected diet composition and energy-balance status independent of changes in body composition.