Valuable but confusing information on serum cholesterol has been derived from epidemiologic dietary studies. The sharp fall in the death rate from circulatory disease in many European countries during World War II was associated with a decreased consumption of milk, butter, and eggs. However, this decrease in fat consumption was coincident in Norway with a decreased protein consumption, as well as a decreased incidence of schizophrenia and suicide. Moreover, in Britain, the death rate from ischemic heart disease began to rise in 1943, although rationing was stringent until 1946. Similar studies are difficult to evaluate. Elaborate epidemiologic surveys have been done in South Africa, Italy, Spain, Guatemala, and Japan, relating the incidence of ischemic heart disease to serum cholesterol level and diet. Generally, these studies suggest that the level of serum cholesterol and the level and nature of dietary fat have a bearing on the incidence of ischemic heart disease.