A study of blood lipids and platelet functions was conducted in north-eastern (Moselle) and southeastern (Var) France, among populations with widely different dietary habits and coronary disease mortality rates. Striking variations were found between the two populations with regard to platelet coagulation activity and response to various aggregation agents. On the other hand, plasma cholesterol, HDL-cholesterol and triglyceride levels were identical. Voluntary reduction by 53 subjects from Moselle in the consumption of saturated fats resulted, within one year, in remarkable improvement in platelet functions, thus demonstrating that the differences were due to diet and not to genetic factors. This being established, two studies were undertaken comparing healthy, normolipaemic subjects (control group) with two groups of type IIb hyperlipaemic subjects following the same pre-established diet and either untreated or treated with fenofibrate. They showed platelet hyperaggregation response to epinephrine in untreated subjects, whereas platelet functions in fenofibrate-treated subjects were identical with those in the control group. It would therefore seem that for many coronary disease patients changes in dietary habits combined with fenofibrate treatment might be one of the simplest and quickest ways of preventing the recurrence of myocardial infarction.