Platelet functions and actions of the antiplatelet drugs aspirin, sulphinpyrazone and dipyridamole are described. The use of antiplatelet drugs in certain thrombotic disorders is surveyed. It is recommended that: (1) Patients with unstable angina should receive aspirin. (2) Aspirin or dipyridamole should be prescribed for at least a year following coronary artery bypass surgery. (3) Patients with prosthetic valve implants should receive an oral anticoagulant. Dipyridamole may increase the effectiveness of the anticoagulant without increasing the bleeding risk.