Components of daily metabolic rate (thermogenesis, BMR and net exercise) were compared between 16 women predisposed to obesity (post-obese) and 16 naturally lean controls of matching age, weight and height, at three levels of activity, in a whole-body respirometer. At all levels of activity, the mean metabolic rate of the post-obese was 15 per cent lower than that of the lean controls. Expenditure on net exercise showed the same relationship, but BMR was only 10 per cent lower, while thermogenesis was 50 per cent lower. The latter was partly due to the smaller food intake of the post-obese and also to a lower thermogenic response. In absolute terms BMR accounted for less than half of the difference in total energy expenditure between the post-obese and the lean (45 per cent). Thermogenesis accounted for approximately 40 per cent of the difference, and 15 per cent after adjusting for the different energy intakes. Significantly more post-obese subjects had a family history of obesity (88 per cent) than lean subjects (38 per cent). Within the post-obese and lean groups there was a consistent trend at each level of activity for those with a family history to have lower metabolic rates, indicating that family history of obesity has an influence on energy expenditure over and above personal history of obesity.