Twenty-nine women who claimed that they could not lose weight on prescribed diets were isolated in a country house and fed 1500 kcal. per day for 3 weeks. Nine women maintained weight within plus or minus 1 kg. and were characterised by low basal (B.M.R.) and daily metabolic rates and by a long previous history of dieting. The remaining women did lose weight, but there was much individual variation. B.M.R. is a good indicator of probable weight loss as a result of dieting. In the group as a whole it was more closely related to body fat than to lean body mass. It was also related to fat-cell number as determined by biopsy, but not to fat-cell size. These findings suggest that among a group of would-be slimmers who claim to be unable to lose weight there will be some who have become metabolically adapted to a low-energy diet and others whose inability to lose weight is illusory.