Genetic tests for adult-onset disorders, including common forms of cancer, are now commercially available, and tests for genetic polymorphisms that predict drug effects or toxicity after treatment are under development. For each of these circumstances, testing of 1 individual may imply an increased risk to his/her relative. The obligation, if any, to warn family members of the identification of a genetic mutation has generated concerns regarding the conflict between the physician's ethical obligations to respect the privacy of genetic information vs the potential liabilities resulting from the physician's failure to notify at-risk relatives. A duty to warn relatives about risks due to some infectious agents has been assumed by state and local health agencies, and the duty to breach confidentiality to warn of imminent harm has been the subject of case law. In general, the special nature of genetic tests has been viewed as a barrier to physicians' breaching the confidentiality of personal genetic information. However, the failure to warn family members about hereditary disease risks has already resulted in 3 lawsuits against physicians in the United States. While the findings of case law and the state and federal statutes that bear on the issue of "duty to warn" of inherited health risk are still being defined, we believe that health care professionals have a responsibility to encourage but not to coerce the sharing of genetic information in families, while respecting the boundaries imposed by the law and by the ethical practice of medicine.