It has been suggested that sufficient fiber in the diet will tend to prevent excessive food intake and depot fat accumulation by decreasing the caloric density of the diet, stowing rate of food ingestion, increasing the effort involved in eating, promoting intestinal satiety, and interfering slightly with efficiency of energy absorption. The increase in the prevalence of obesity in Western countries since 1900 has taken place concurrently with marked changes in the nature of the diet. Per capita intake of dietary fiber associated with starchy foods has greatly decreased, but intake of fiber associated with fruits and green vegetables has increased. Thus, although the type of fiber in the diet has changed, the total quantity may not have diminished considerably. Studies of the effect of caloric dilution with cellulose and other metabolically inert bulking agents have disclosed little or no inhibitory effect on the spontaneous energy intake of nonobese laboratory animals and human subjects. Nevertheless, there is evidence that obese rats and humans may defend their excess weight against nutritive dilution with less tenacity than their nonobese counterparts. The hypothesis that dietary fiber can protect against obesity therefore deserves further testing since an increase in the fiber content of the diet may tend to prevent overeating and excessive weight gain even if it does not reduce spontaneous energy intake in nonobese organisms.