While twentieth-century medical ethics has focused on the duty of physicians to benefit their patients, the next century will see that duty challenged in three ways. First, we will increasingly recognize that it is unrealistic to expect physicians to be able to determine what will benefit their patients. Either they limit their attention to medical well-being when total well-being is the proper end of the patient or they strive for total well-being, which takes them beyond their expertise. Even within the medical sphere, they have no basis for choosing among the proper medical goals for medicine. Also, there are many plausible strategies for relating predicted benefits to harms, and physicians cannot be expert in picking among these strategies. Second, increasingly plausible ethical systems recognize that in some cases, patient benefit must be sacrificed to protect patient rights including the right to the truth, to have promises kept, to have autonomy respected, and to not be killed. Third, ethics of the next century will increasingly recognize that some patient benefits must be sacrificed to fulfill duties to others - either the duty to serve the interests of others or other duties such as keeping promises, telling the truth, and, particularly, promoting justice. Physicians in the twenty-first century will be seen as having a new, more limited duty to assist the patient in pursuing the patient's understanding of the patient's interest within the constraints of deontological ethical principles and externally imposed duties to promote justice. The result will be a duty to be loyal to the consumer of health care with the recognition that often this will mean that the physician is not permitted to pursue the physician's understanding of the patient's well-being.