An understanding of the international normalized ratio (INR)--which was developed to standardize reporting of the prothrombin time (PT) and provide consistent regulation of anticoagulation--is important. The recommended therapeutic range for the INR (which is calculated from the patient's PT, a mean control PT, and the international sensitivity index) for oral anticoagulant treatment of most conditions is 2.0 to 3.0. In patients with mechanical cardiac valves, the INR should be at least 2.5 to 3.5. A common cause for progression of venous thromboembolic disease and treatment failure is inadequate heparinization during the first day of treatment. Therefore, an intravenous bolus of 5,000 to 10,000 U of heparin should be administered before a maintenance infusion is initiated. Also during the first day of treatment, warfarin therapy can be implemented. Overlap treatment with heparin and warfarin for 4 or 5 days is recommended. Low-molecular-weight heparins, a new class of anticoagulants, have been shown to be more effective than standard heparin in preventing venous thrombosis in orthopedic surgical patients, but at a higher cost. Patients with mechanical cardiac valves who are receiving anticoagulant therapy and are scheduled for noncardiac operations must have a risk-to-benefit assessment of the need for continuous anticoagulation performed preoperatively. Many of these patients can safely discontinue warfarin therapy for several days as outpatients before the surgical procedure. Preoperative heparin therapy and warfarin withdrawal in the hospital are recommended only for those patients with cardiac valves at high risk for systemic embolization (with a mitral valve prosthesis, cardiomyopathy, or previous thromboembolism). The concurrent use of certain drugs or presence of comorbid conditions can predispose to hemorrhagic complications of anticoagulant therapy.(ABSTRACT TRUNCATED AT 250 WORDS)