The role of platelets in the initation of arterial thrombosis is clear. In venous thrombosis as well, platelets may in some circumstances play a significant role. For these reasons, and because of the complications and limitations of anticoagulant therapy, antithrombotic trials have been launched with several agents which inhibit platelet function. Regarding postoperative deep vein thrombosis, neither aspirin nor dipyridamole alone appears effective, although the combination offers promise. Results with the dextrans are conflicting. In recurrent idiopathic deep vein thrombosis, sulfinpyrazone may be beneficial. On the arterial side, transient cerebral ischemic attacks may be favorably affected by either aspirin or sulfinpyrazone. Prevention of thromboembolism associated with prosthetic heart valves appears possible with combination warfarin dipyridamole therapy, and the beneficial effect of sulfinpyrazone on shortened platelet survival in this group suggests that this agent may also be effective. Sulfinpyrazone may also be beneficial in preventing thrombosis in arteriovenous canulas. The issue which has attracted the greatest attention and about which no clear answer exists at present is whether antiplatelet agents can modify the course of acute myocardial infarction. Several trials with aspirin are currently underway, and it would be premature to recommended its use in this condition until the results of these trials are available, probably in 1975.