Abstract Cholesterol metabolism was studied during the ingestion of 3 diets differing only in the degree of saturation of their fat contents. The iodine numbers of fats in the diets were 44.8 (diet I), 131 (diet II) and 10 (diet III). On changing from diet I to diet II there was a considerable increase in rate of excretion of sterols in feces and in sterol concentrations in bile. On changing from diet II to diet III there was a progressive decrease in fecal excretion of cholesterol metabolites. Thus, relative saturation-unsaturation of dietary fat appears to modify the excretory pattern of cholesterol and its metabolites. Prompt diet-induced changes in concentration of cholesterol and its metabolites in bile indicate the need for further evaluation of the quantitative aspects and physiologic significance of the mechanisms involved.