In rats, diets rich in polyunsaturated fatty acids of the (n-6) and (n-3) families (P) reduce the tendency to arterial thrombosis, whereas diets containing long-chain saturated fatty acids (S) are prothrombotic. Monounsaturated fatty acids seem to have a neutral influence on arterial thrombus formation. Consequently, the P/S ratio is an important determinant of the thrombogenicity of a dietary fat, the effect of which is probably mediated by an influence on platelet aggregability. In man, the type of dietary fat also affects platelet aggregation. Methodological problems, however, have resulted in equivocal results from studies, especially with respect to diets enriched with fish oil. Therefore, more research is required with methods than can quantify thrombotic risk in a reliable way. Nevertheless, the available evidence already justifies an active policy to increase people's consumption of (poly)unsaturated fatty acids at the expense of long chain saturated fatty acids.