The association in many populations of the fat content of the diet and frequency of ischemic heart disease is mentioned. The author gives a summary of papers showing on one hand, a high correlation between the content of the diet in saturated fatty acids and ischemic heart disease morbidity and mortality rates, and, on the other hand, the protective effect of a change towards more polyunsaturated fatty acids in the diet. Several teams of Belgian researchers have recently published new facts on this matter. In the Northern Flemish speaking part of the country, saturated fatty acids have been partially replaced by polyunsaturated fatty acids from vegetable sources. This change has been followed by a general decrease of ischemic heart disease incidence, prevalence and mortality rates. In the Southern French speaking part of the country, the alimentary changes have not occurred so far; the ischemic heart disease morbidity and mortality rates stand higher.