The resting metabolic rates (R.M.R.s) of 69 obese patients were measured during a period of weight stability. All except 4 women had a rate, in megajoules per 24 hours, in excess of that for subjects of normal weight. This increased R.M.R. is obscured by the traditional method of expressing R.M.R. per unit surface area. The high R.M.R. in the obese state was related not to the excess fat but to a 36% and 32% increase in the lean body mass of the men and women respectively. The R.M.R.s of 30 patients measured during weight-loss fell. The increase in R.M.R. in obesity is an important mechanism for achieving energy balance, whereas the progressive fall in R.M.R. during slimming demonstrates the need for a permanent reduction in food intake if energy balance is to be maintained on reaching normal weight. Measuring only the R.M.R. in the obese state is unlikely to help in understanding the pathogenesis of obesity.