Ethical principles that require the preservation of patients' confidential information are reinforced by principles found in several areas of law, such as law on contracts, negligence, defamation and fiduciary duty. However, laws sometimes compel disclosures of medical confidences, and more often may justify or excuse disclosures. Legally contentious issues concern patients' confidences regarding possible unlawful conduct, such as pregnancy termination, and the risk of spread of HIV and other infections. This article reviews the various legal bases of the duty of confidentiality, and legal challenges to the ethical obligation of:nondisclosure. It addresses the justifications and limits of exchange of patients' health information among healthcare professionals and trainees, and considers legally recognized limits of confidential duties, and the scope of legitimate disclosure. An underlying theme is how to determine whether physicians are ethically justified in employing the discretion the law sometimes affords them to breach patients' expectations of confidentiality. (C) 2000 International Federation of Gynecology and Obstetrics.