In certain instances the law recognizes that a person’s free will may be so influenced by another that agreements made by that person should be set aside. The primary philosophical basis for this doctrine of ‘undue influence’ is respect for individual autonomy and the need to ensure that people have freely chosen a course of action, before the law will make that course of action binding upon them. In the past, the doctrine of undue influence was confined to situations involving the transfer of property (such as gifts, wills or purchases). But the doctrine is also relevant to consents to medical treatment, most commonly in cases where a patient refuses treatment because of the influence of family members. The purpose of this article is to provide a general outline of the legal concept of undue influence and then to consider its recent application in medical lawsuits.