Diets involving a reduction in caloric intake are frequently prescribed for the treatment of obesity, but their long-term efficacy is questionable. We considered a calorie restricted diet successful if the weight loss was ≥ 5% after at least 3 years follow up. From published data, calculating a definitive percentage of successful cases is difficult because of the way data are presented and because loss to follow-up is not corrected for in many studies. Judging by the best data available, the success rate is very low. Most individuals will regain weight and sometimes even more than they lost in the first place. The mechanisms driving this weight increase are a decrease in energy expenditure and an increased appetite which is mediated by factors such as leptin. If the first attempt to lose weight fails, the advice to go on a diet should not be endlessly repeated; stabilizing the individual's weight would probably be a more realistic goal.