Numerous controlled dietary experiments in various U.S. laboratories have demonstrated that fatty acid composition and cholesterol content of the diet influence human serum cholesterol concentration. Work in Ancel Keys's (1) laboratory has shown that the serum cholesterol changes produced by controlled dietary modifications in middle-aged, metabolically normal male subjects in energy balance can be predicted from changes in fatty acid composition and cholesterol content of the diet. The serum cholesterol changes are mostly accounted for by those of low-density lipoprotein cholesterol. The results of some of the European studies are generally comparable to those obtained in Minnesota. Thus, the results in one of the Finnish Hospital studies are in perfect agreement with the prediction of the Keys, Anderson, and Grande equation: good agreement has also been found with the results of a recent experiment by Schlierf et al. in Heidelberg. The effects of dietary cholesterol observed by Dutch workers are also in agreement with our prediction, when cholesterol (egg yolk) was added to a diet rich in polyunsaturated fatty acids, but not when added to a diet rich in saturated fatty acids.