Harm caused by the failure of health professionals to warn an at-risk genetic relative of her or his risk is genetic harm. Genetic harm should be approached using the usual principles of negligence. When these principles are applied, it is shown that (a) genetic harm is foreseeable; (b) the salient features of vulnerability, the health professional's knowledge of the risk to the genetic relative and the determinancy of the affected class and individual result in a duty of care being owed to the genetic relative; (c) the standard of care required to fulfil the duty to warn should be the expectations of a reasonable person in the position of the relative; and (d) causation is satisfied as the harm is caused by the failure of intervention of the health professional. Legislation enacted subsequent to the Report of the Commonwealth of Australia, Panel of Eminent Persons (Chair D Ipp), Review of the Law of Negligence Report (2002) and relevant to a duty to warn of genetic harm is considered. The modes of regulation and penalties for breach of any future duty to warn of genetic harm are considered.