SummaryMany mechanisms may be involved in the role of platelets in thrombogenesis and therefore more than one platelet blocking agent may be required to prevent thrombosis. However, the action of blocking agents must be established independently before they are studied in combination.The action of one agent–aspirin (which, in fact, blocks the platelet release action and does not interfere with primary aggregation)–is studied more closely here in four clinical conditions: venous thrombosis, arterial thrombosis, atherosclerosis, and in one case of multiple chronic microemboli with resultant intensive lung destruction. The evidence obtained so far is not conclusive, and more research is needed, but the future of this, and other, anti-platelet agents looks promising.