The present text is the revised/updated version of the CIOMS International Ethical Guidelines for Biomedical Research Involving Human Subjects. It consists of 21 guidelines with commentaries. A prefatory section outlines the historical background and the revision process and includes an introduction an account of earlier instruments and guidelines a statement of ethical principles and a preamble. An Appendix lists the items to be included in the research protocol to be submitted for scientific and ethical review and clearance. The Guidelines relate mainly to ethical justification and scientific validity of research; ethical review; informed consent; vulnerability - of individuals groups communities and populations; women as research subjects; equity regarding burdens and benefits; choice of control in clinical trials; confidentiality; compensation for injury; strengthening of national or local capacity for ethical review; and obligations of sponsors to provide health-care services. They are designed to be of use to countries in defining national policies on the ethics of biomedical research involving human subjects applying ethical standards in local circumstances and establishing or improving ethical review mechanisms. A particular aim is to reflect the conditions and the needs of low-resource countries and the implications for multinational or transnational research in which they may be partners.