In 2001, the International Rett Syndrome Association funded the establishment of a World Wide Web-based database to collect and display the genetic data of children and adults with Rett syndrome from around the world. RettBASE (<http://mecp2.chw.edu.au>) encompasses both published and unpublished data; includes pathogenic mutations, benign polymorphisms, and sequence variations of uncertain significance; and has a range of query capabilities, allowing for simple or complex interrogation of the database. To undertake genotype-phenotype correlations and to identify the likely subtle differences in phenotype, detailed phenotype data on large samples will be provided by the International Rett Syndrome Association International Phenotype database InterRett. InterRett is under development by the Australian Rett syndrome study group at the Telethon Institute for Child Health Research in Perth, Western Australia. It will collect data from clinicians and families and provide deidentified, collated data on the Internet (<http://www.ichr.uwa.edu.au/rett/irsa>). Data records will be linked with RettBASE through a common unique identifier. An international reference panel is advising on the development of the database. Data collection procedures from families and clinicians are currently being piloted. Full data collection from both groups began in the second half of 2003. Concurrently, the output database will be developed to provide deidentified individual records and collated data for clinicians and researchers and collated data for families and the general public. This Web-based database will be an invaluable resource for understanding the nature of the disorder and managing children and adults with Rett syndrome.