[Registration of observational studies: it is time to comply with the Declaration of Helsinki requirement]

Publication bias is a serious deficiency in the current system of disseminating the results of human research studies. Clinical investigators know that, from an ethical standpoint, they should prospectively register clinical trials in a public registry before starting them. In addition, it is believed that this approach will help to reduce publication bias. However, most studies conducted in humans are observational rather than experimental. It is estimated that less than 2% out of 2 million concluded or ongoing observational studies have been registered. The 2013 revision of the Declaration of Helsinki requires registration of any type of research study involving humans or identifiable samples or data. It is proposed that funding agencies, such as the Fondo de Investigaciones Sanitarias, as well as private companies, require preregistration of observational studies before providing funding. It is also proposed that Research Ethics Committees which, following Spanish regulation, have been using the Declaration as the framework for assessing the ethics of clinical trials with medicines since 1990, should follow the same provisions for the assessment of health-related observational studies: therefore, they should require prospective registration of studies before granting their final approval. This would allow observational study investigators to be educated in complying with an ethical requirement recently introduced in the most important ethical code for research involving humans.