The effect of a moderate energy intake plus exercise (MEEX) vs a low energy intake without exercise (LESD) on weight loss and energy expenditure was examined in two groups of moderately overweight women confined to a metabolic unit. An initial 2-week baseline period was used to determine weight maintenance energy requirement (ER). This was followed by a 6-week weight reduction period. Energy intake was decreased to 0.75 ER in MEEX and treadmill exercise was individually prescribed to increase energy expenditure to 1.25 ER. Energy intake was decreased to 0.5 ER in LESD. Thus, both energy intake and expenditure were manipulated to result in an energy deficit of 50 percent. Although total weight loss was significantly higher in LESD compared to MEEX (7.8 vs 5.7 kg), fat loss did not differ (5.1 vs 4.7 kg) hence the proportion of weight loss due to fat was greater in MEEX. Aerobic capacity, measured by maximum oxygen uptake, significantly improved in MEEX (2.44 to 2.84 l/min) but did not change in LESD (2.51 to 2.50 l/min). Basal metabolic rate, energy cost of standardized activities, and 3-hour thermic response to a test meal did not change in either group throughout the study. Thus, a 50 percent energy deficit, achieved by either diet alone or diet in combination with exercise, did not result in conservation of energy expenditure in moderately overweight women.