We argue that if a terminally ill patient wants to know the truth about his or her condition, and the patient's doctor possesses that information, then the doctor is morally obliged to tell the truth. The argument is not based on any supposed right to information (even information about oneself); nor on any supposed moral obligation to tell the truth; nor on any supposed property rights the patient might have. Rather, the argument is based on the very principle usually invoked to license deception: the harm principle (that doctors ought to take that course of action which does least harm to the patient).