For rare diseases, clinical and epidemiological research suffers from very small numbers of cases. A comprehensive collection of data and information in registries is an essential precondition to improve this situation. To this end, a number of disease specific networks have started collecting data with support from the German Ministry of Research. The past experiences of the German Childhood Cancer Registry show that voluntary participation, based on informed consent, can result in a satisfactory completeness of data collection and, thus, enable successful medical research. There are several ways to build registries and research networks conforming to the data protection rules.