Registries of clinical trials are a potentially useful resource for the planning of new studies, the promotion of communication and collaboration between researchers, the conduct of meta-analyses, and the facilitation of patient access and recruitment to trials. However, many physicians and researchers are unaware of their existence, and as a result they remain underused. A directory of registries of clinical trials has been developed as a result of an international survey of 63 organizations and 51 individuals in 13 different countries to identify the existence of such registries. This is intended as a resource to keep physicians, researchers, trial organizers, funding bodies and government agencies abreast of the growing number of registries available, and to assist in the planning of future registries. Twenty-four current and six planned registries of clinical trials have been identified. Most focus on AIDS or oncology, but such diverse areas as neurosurgery, cardiovascular disease, dentistry and perinatology are also represented. This paper presents a descriptive profile on each registry and discusses the relative merits of their different organizational features. Recommendations are given for the establishment of future registries.