Twenty-four hour energy expenditure (24EE) can be measured in a respiratory chamber. 24EE is comprised of the basal metabolic rate, the thermic effect of food, and the energy cost of physical activity. The major determinant of 24EE, fat-free mass, accounts for approximately 80% of the variance observed between individuals. Genetic factors seem to be the cause of the familial aggregation of 24EE in man. The variability of 24EE for a given body size and composition is of importance because a low metabolic rate is a major risk factor for weight gain in man. There is increasing evidence that obesity, often an inherited disorder, cannot always be attributed to gluttony and sloth. Similar to the need to treat essential hypertension, there is a need to treat a disorder perhaps best called essential obesity.