Edmund Pellegrino has argued that the dramatic changes in American health care call for critical reflection on the traditional norms governing the therapeutic relationship. This paper offers such reflection on the obligation to "do no harm." Drawing on work by Beauchamp and Childress and Pellegrino and Thomasma, I argue that the libertarian model of medical ethics offered by Engelhardt cannot adequately sustain an obligation to "do no harm." Because the obligation to "do no harm" is not based simply on a negative duty of nonmaleficence but also on a positive duty of beneficence, I argue that it is best understood to derive from the fiduciary nature of the healing relationship.