Oral anticoagulant therapy may be inappropriately managed and thus potentially place the warfarin-treated patient at an increased and unnecessary risk of bleeding or thromboembolic complications. This management issue is largely due to variations in measuring the pro-thrombin time. These variations may lead to an inaccurate and inconsistent assessment of the patient's true level of anticoagulation. The adoption of the International Normalized Ratio (INR) is recommended because it provides a mathematical correction for one of these variations—specifically, the thromboplastin reagents currently used. This article focuses on the appropriate use of the INR to monitor patients' responses to warfarin sodium therapy. It is vitally important for health care providers to be aware of the INR, to use it in clinical practice, and to interpret it correctly.