It is not known whether the decrease in the thermic effect of food (TEF) in obesity is a consequence of obesity or a factor contributing to the development of obesity. The resting energy expenditure (REE) of 24 obese, nondiabetic, postmenopausal women was 5481 ± 110 kJ/24 h (1310 ± 26.4 kcal/24 h). After weight loss (12.7 ± 0.45 kg) the REE was significantly decreased (4858 ± 94 kJ/24 h, or 1161 ± 22.4 kcal/24 h) and equivalent to the REE of 4866 ± 119 kJ/24 h (1163 ± 28.5 kcal/24 h) in 24 never-obese, postmenopausal women. The TEF, expressed as a percentage of the calories ingested, was 8.2 ± 0.50% for obese subjects, 8.7 ± 0.57% for postobese subjects, and 9.8 ± 0.54% for never-obese subjects. Compared with never- obese subjects, the TEF was significantly reduced in obese subjects (P = 0.043) and remained unchanged after weight loss (P = 0.341). These findings indicate that the lower TEF in the obese subjects is uncorrected by weight loss, and thus it is a contributor to obesity rather than a consequence of obesity.