The oxygen consumption of human adipose tissue (AT) was determined in 53 adults, lean and obese, and in nine lean boys. The oxygen consumption was positively related to fat cell weight and negatively to age and degree of obesity. Men and women did not differ with respect to oxygen consumption of AT. The positive relationship between oxygen consumption per cell and fat cell size was also demonstrated in size-separated cells from the same donors. Expressed per cell the oxygen consumption was higher in fat cells from obese than in cells from lean subjects, but expressed per gram of tissue the opposite result was found. The oxygen consumption of the total AT organ was higher in obese than in lean subjects. The energy expenditure of AT constituted approximately 4% of the estimated 24-h energy expenditure in both groups. It is concluded that obese subjects do not maintain their obesity because of a reduced energy expenditure of the total AT (or of the total body). After a partial weight reduction in five subjects, the energy metabolism tended to change in direction toward the conditions seen in lean subjects. However, it is still an open question whether the observed energy metabolic aberrations of obese human AT are only secondary to the obese state or partly primary and thus of etiological importance.