The percentage of energy from fat in diets has been thought to be an important determinant of body fat, and several mechanisms have been proposed. Comparisons of diets and the prevalence of obesity between affluent and poor countries have been used to support this relationship, but these contrasts are seriously confounded by differences in physical activity and food availability. Within areas of similar economic development, regional intake of fat and prevalence of obesity have not been positively correlated. Randomized trials are the preferable method to evaluate the effect of dietary fat on adiposity and are feasible because the number of subjects needed is not large. In short-term trials, a modest reduction in body weight is typically seen in individuals randomized to diets with a lower percentage of calories from fat. However, compensatory mechanisms appear to operate, because in randomized trials lasting >or=1 year, fat consumption within the range of 18% to 40% of energy appears to have little if any effect on body fatness. The weighted mean difference was -0.25 kg overall and +1.8 kg (i.e., less weight loss on the low-fat diets) for trials with a control group that received a comparable intensity intervention. Moreover, within the United States, a substantial decline in the percentage of energy from fat during the last 2 decades has corresponded with a massive increase in the prevalence of obesity. Diets high in fat do not appear to be the primary cause of the high prevalence of excess body fat in our society, and reductions in fat will not be a solution.