There is a narrow line between the benefits and risks of anticoagulant therapy. Many factors influence a patient’s response to warfarin, and careful monitoring is required to ensure that the therapeutic level of anticoagulation is achieved. The purpose of this retrospective review was to examine the relationship between the postoperative response to warfarin and the preoperative level of haemoglobin. The results showed that lower preoperative levels of haemoglobin are associated with an increased response to warfarin (p = 0.01).