Since every person has the right to determine what will be done to his or her body, he or she has the right to decide whether or not to undergo medical treatment. If this decision is to be more than a pure formality, the patient needs to be fully informed of what that decision entails, and so has a right to know of the risks involved in the treatment he or she is considering. A physician has a corresponding duty to impart the information which the patient needs to enable him or her to reach such an informed decision. This article traces developments in common-law and civil law jurisdictions and considers the extent to which they protect the patient's right to know. The comparative law analysis reveals that English law has tended to fall behind both its common-law relatives and its European neighbours in the amount of protection it affords to this fundamental right because it has allowed liability to be determined by a negligence standard which treats a physician's conformity with the practice of a body of medical opinion as conclusive evidence that he or she has discharged his or her duty. The article warns of a further threat to the patient's right to make an informed decision which has arisen in other common-law jurisdictions in the guise of the so-called 'reasonable patient', whose abstract nature means that his or her presence in standard of care and causation questions brings with it an evidential void which tends to be filled by the evidence of medical experts so that a physician may, once again, be relieved from liability even though he or she has failed to disclose information that the patient before him or her needed to know for the purposes of a treatment decision. The conclusion to be drawn is that only where the standard of care is based on the needs of each patient rather than the opinion of a body of doctors, and only where the focus is kept on the actual patient rather than the hypothetical 'reasonable patient' is the patient's right to know properly protected.