In rats, diets rich in polyunsaturated fatty acids of the (n-6) and (n-3) families (P) lower arterial thrombosis tendency, whereas long-chain saturated fatty acids (S) are prothrombotic. Monounsaturated fatty acids seem to behave neutrally as to 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 dietary fat type also affects platelet aggregation. Methodological problems, however, resulted in equivocal results, especially with respect to diets enriched with fish oil. Thus, more research is required with methods that are able to quantify thrombotic risk in a reliable way. Nonetheless, the available evidence already justifies an active policy to increase the consumption of (poly)unsaturated fatty acids at the expense of long-chain saturated fatty acids.