Chelation therapy originally was administered exclusively to patients with heavy metal poisoning. Now some physicians are administering this therapy for numerous conditions, most commonly coronary heart disease. A 64–year-old man experienced impaired warfarin anticoagulation after undergoing chelation therapy. His international normalized ratio (INR) fell from 2.6 the day before to 1.6 the day after therapy was administered. Whether chelation therapy decreases the effectiveness of warfarin anticoagulation is uncertain. However, because of this potential interaction, clinicians should consider increased INR monitoring in patients undergoing chelation therapy.