There is growing concern that dieting may adversely affect the metabolic rate and exacerbate efforts to control weight. In this study we measured the resting metabolic rate nine times over 48 weeks in 18 obese women (108.0 ± 3.1 kg) who were randomly assigned to one of two dietary conditions. Nine patients consumed approximately 5021 kJ/d (1200 kcal/d) throughout the 48 weeks, while the other nine consumed a 1757-kJ/d (420-kcal/d) diet for 16 of the first 17 weeks and a conventional reducing diet for the remainder of treatment. All patients increased their physical activity, primarily by walking. During the first 5 weeks, the fall in metabolic rate was more than double the relative reduction in weight. By contrast, at week 48, the metabolic rate of patients in the two conditions was reduced by 9.4% ±4.0% and 8.3% ±2.2%, respectively, while weight was reduced by 16.6% ±2.7% and 19.5%±2.7%, respectively. Thus, neither dietary regimen, combined with modest physical activity, was associated with long-term reductions in resting metabolic rate that exceeded decreases anticipated with the achievement of a lower body weight.