Flutamide, a chemotherapeutic agent for prostate cancer, is known to enhance warfarin anticoagulation. However, not much is known about its pharmaceutical interaction. We herein report the case of a patient with an implanted pacemaker for atrial fibrillation with bradycardia who was on warfarin. This patient presented with deterioration of hematuria, gingival, ear, and subcutaneous bleeding. The prothrombin time-international normalized ratio was extremely elevated after starting flutamide to treat progression of prostate cancer. Fatal bleeding complications were able to be prevented by the immediate administration of prothrombin complex concentrate, but the effect of flutamide on warfarin was prolonged for about two more weeks after the withdrawal of flutamide.