It is broadly held that confidentiality may be breached when doing so can avert grave harm to a third party. This essay challenges the conventional wisdom. Neither legal duties, personal morality nor personal values are sufficient to ground professional obligations. A methodology is developed drawing on core professional values, the nature of professions, and the justification for distinct professional obligations. Though doctors have a professional obligation to prevent public peril, they do not honor it by breaching confidentiality. It is shown (...) how the protective purpose to be furthered by reporting is defeated by the practice of reporting. Hence there is no conflict between confidentiality and the professional responsibility to protect endangered third parties. *A longer version of this article will appear as “Medical Confidentiality” in The Blackwell Guide to Bioethics, ed. R. Rhodes et al. Oxford, UK: Blackwell, 2006. (shrink)