The registration of clinical trials has been the focus of attention within the medical literature,1–4 as well as in the lay press and the US Congress.5–6 Clinical trials registries are Web-based databases of clinical trial information that serve both ethical and scientific functions. Registries serve the ethical function of ensuring that the public has information about ongoing and previously conducted trials. Registries also provide researchers, journal editors, and reviewers with the context for understanding research results; by providing a complete list of clinical studies, they can alert researchers to studies that do not have published results. Trial registries differ with regard to a number of criteria, including the sponsoring organization, the focus (general vs disease specific), and the trial information that they contain. Many groups, including the International Committee of Medical Journal Editors (ICMJE), endorse the principles that registries should be managed by a nonprofit organization and should be free of charge for both registrants and users. Trial registries differ from results databases: the former generally include protocol and recruitment information, whereas the latter include study results.