During recent years a considerable improvement in diagnostic techniques has enabled cytogeneticists to find more and smaller chromosomal aberrations. However, accurate clinical knowledge about rare chromosome disorders is frequently lacking, mostly due to a significant decline in publishable cases. On the other hand, there is an increasing demand from parents and physicians for reliable information. In order to improve the quality and the quantity of data available, we designed a new database named the European Cytogeneticists Association Register of Unbalanced Chromosome Aberrations (ECARUCA) at http://www.ecaruca.net. This Internet-database contains cytogenetic and clinical data of patients with rare chromosome abnormalities, including microscopically visible aberrations, as well as microdeletions and -duplications. Cases with certain breakpoints collected in the Zurich Cytogenetic Database were transferred to ECARUCA. The advantages of ECARUCA compared to existing sources are that ECARUCA is interactive, dynamic and has long-term possibilities to store cytogenetic, molecular and clinical data. Professionals can login to submit new cases and perform searches in the database through the Internet. Currently the database contains 1500 unique chromosomal aberrations from almost 4000 patients. A frequent submission of new data ensures the up-to-date quality of the collection. Individual parent accounts allow parents to inform the ECARUCA team about the follow-up of their child. The ECARUCA database provides health care workers with accurate information on clinical aspects of rare chromosome disorders. Additionally, detailed correlations between chromosome aberrations and their phenotypes are of invaluable help in localising genes for mental retardation and congenital anomalies.