A summary of the recommendations for antithrombotic therapy after myocardial infarction appears in Table 1. The American College of Cardiology/American Heart Association Task Force used a three-tiered classification of therapeutic interventions after myocardial infarction. Class I includes interventions that are usually indicated, always acceptable, and considered useful and effective. Class II includes treatments that are considered acceptable but are of uncertain efficacy and possibly controversial. This class is further subdivided into class IIa (weight of evidence in favor of usefulness and efficacy) and IIb (not well established by evidence, can be helpful, and probably not harmful). Class III includes interventions that are not indicated and possibly harmful. None of the antithrombotic therapies under consideration in this review were in Class III; therefore, this category does not appear in Table 1. Contraindications to anticoagulation must, however, be considered before anticoagulant therapy is started. To prevent arterial embolism, immediate anticoagulation with heparin sufficient to prolong the activated partial thromboplastin time to 1.5 to 2.0 times control should be initiated in patients with large anterior myocardial infarctions. This should be followed by warfarin therapy for at least 3 months in patients with anterior or apical wall-motion abnormalities or demonstrated mural thrombus. Indefinite oral warfarin therapy may be considered in patients with diffusely dilated and poorly contracting left ventricles. For the prevention of reocclusion after thrombolytic therapy, aspirin and heparin should be initiated immediately. Aspirin should be continued indefinitely; the heparin may be discontinued after 24 to 72 hours.(ABSTRACT TRUNCATED AT 250 WORDS)