Many statisticians work with informal codes of ethics, and are probably unaware of the existence or content of rules which have been drawn up to govern statistical practice. Medical statisticians will be aware of codes of conduct for medical research, and most codes of professional ethics have some dependence on evidence. Statisticians, therefore, have a valuable contribution to make to debates on ethics which concern scientific soundness, data and perceptions of risk. A lively debate on the revision of the widely respected Declaration of Helsinki, to reflect issues arising from research in developing countries, particularly HIV research, centres on questions of study design, data analysis and assessment of risk. Collectively owned multiprofessional work requires each of the various professions to take responsibility for the conduct of the research, and the impact that it might have. Statisticians share important responsibilities in maintaining ethical medical research in all countries.