Randomised trials of medical interventions are the foundation of evidence based health care, but most are effectively conducted in secret. Few people—usually an elite network of investigators, funding agencies, and government regulatory bodies—know about a trial from its inception Most trials become public knowledge only when the investigators publish their completed project in a journal—if they ever do.1 In the meantime, others may be duplicating the effort or, worse, ignoring early warning signs that an intervention is dangerous. One trial of the class I antiarrhythmic drug lorcainide, for example, went unreported for over a decade even though the data suggested that the drug increased mortality in patients with myocardial infarction During that time, use of such drugs continued and shortened the lives of up to 70 000 people each year in the United States alone.2 A register of clinical trials is one way of opening up the process and avoiding these problems.

The idea was first mentioned 13 years ago3 and has been refined since then by an international group of trialists, academics, and enthusiasts campaigning for a comprehensive, up to date, and searchable archive of ongoing and recently completed randomised trials, including trials done by the pharmaceutical industry. This article reports a recent conference on trial registration hosted in London by the BMJ, the Lancet, and the Association of the British Pharmaceutical Industry. The article was prepared from conference presentations and subsequent debate, discussion with selected speakers and delegates, reviews by those speakers who are running their own registers, and a bibliography of studies prepared by Iain Chalmers, director of the UK Cochrane Centre.