Resting metabolic rate (RMR) was determined in 10 obese and 10 nonobese women after an overnight fast and for 3 h after the ingestion of an 800-kcal liquid meal. In the fasting state, absolute energy expenditure in the obese (4.8 +/- 0.2 kJ/min) was 25% greater than in the nonobese (P less than 0.005), but was comparable with the nonobese when expressed in relation to body surface area or lean body mass and was reduced by 20% when expressed per kilogram body weight3/4 (P less than 0.005). Meal ingestion resulted in a 14-16% increase in RMR (postprandial thermogenesis) that was similar in the two groups, so that absolute energy expenditure in the obese remained 22-25% higher than in the nonobese throughout the postprandial period. The estimated overall (fasting and postprandial) increase in resting caloric expenditure in the obese as compared with the nonobese was 350-375 kcal/day.