Prostaglandins potently regulate platelet aggregation and vascular tone. Thus they are of great importance in the blood supply of tissue. The strategy of the attempts to prevent occlusive vascular disease have been either to decrease the amount of aggregatory, vasoconstricting prostaglandins or to increase antiaggregatory, vasodilatory prostaglandins. Acetylsalicylic acid has been mostly used in attempts to inhibit platelet TxA2-synthesis without disturbing endothelial cell PGI2-synthesis. Primary prevention trials of myocardial infarction have given results of great discrepancy whereas reinfarctions are likely in part to be inhibited by acetylsalicylic acid. In the prevention of ischaemic cerebral disease, most of the trials have indicated the beneficial effect of acetylsalicylic acid, as is the case also in the prevention of venous thrombosis. It is, however, likely that the doses of acetylsalicylic acid have not been optimal in any work, and thus better results are to be awaited. The infusion of anti-aggregatory, vasodilatory prostaglandins, mainly PGI2 or PGE1, have relieved the symptoms of ischaemia of the lower extremities, coronary heart disease and Raynaud's syndrome. The technical difficulties of these infusions, however, limit their applicability to special groups of patients.