Long-term anticoagulant therapy in cardiology practice has been placed on a firm scientific footing based on rigorous clinical trial evaluation. Two factors have markedly improved the efficacy and safety of long-term anticoagulants. These include the demonstration of the efficacy and safety of less intense levels of anticoagulation than previous standards and the introduction of the International Normalized Ratio (INR) to standardize anticoagulant monitoring and control.