We examine the ethical principles underpinning advance directives (ADs) and the legal duties of doctors in determining their validity. A physician attending an incompetent patient with an acute life-threatening illness, and an AD refusing treatment, should ensure that the AD is legally valid before making the treatment decision. Treatment against a patient's wishes, as expressed in a valid AD, compromises patient autonomy and may constitute battery. Conversely, withholding treatment in accordance with an AD that is not legally valid risks substantial harm to the patient and may constitute breach of the duty of care and negligence. Legally valid directives should be respected. If an AD is not legally valid, the patient should be treated in his or her best interests. If uncertain, the physician should treat according to the patient's best interests while seeking legal advice.