Resting metabolic rate, measured by the ventilated hood technique in 11 obese subjects before weight loss was 6.5 +/- 0.3 kJ/min, not significantly different from the value predicted by regression equations relating metabolic rate to body weight (established from measurements in 106 subjects). After 34.5 kg mean weight loss, and during continued dieting, resting metabolic rate fell an average of 25% to 5.0 +/- 0.18 kJ/min, significantly below that predicted for the new slimmed weight. The fall in metabolic rate, over and above that expected from weight loss alone, suggests that the slimmed obese are hypometabolic compared with subjects of the same weight who have not lost weight. The failure of the slimmed obese to maintain their weight loss is unlikely to be due solely to this excessive fall in resting metabolic rate, since they still had quantitatively higher energy expenditure than lean subjects.