In the UK, decisions taken on behalf of patients who lack capacity must be in their best interests, but the meaning of ‘best interests’ has evolved. The Mental Capacity Act 2005 codified the law relating to adults who lack capacity, offering a checklist of factors relevant to best interests assessment, including the person’s ‘past and present wishes and feelings’ and the ‘beliefs and values that would be likely to influence’ their decisions. Since 2013, the courts have gone further and, in some circumstances, seem to treat the patient’s wishes as the decisive factor. Now, a gap exists between what the statute says and its judicial interpretation. Doctors seeking guidance on how to weigh the patient’s own wishes when making a best interests decision will receive divergent advice from the legislation, guidance from the National Institute for Health and Care Excellence (NICE), and the law reports. This chapter suggests this discrepancy is unnecessarily confusing for health care professionals, patients and their families. If there is, or should be, a rebuttable presumption in favour of making the patient’s wishes decisive, the law would preferably state this clearly and unequivocally.