InterRett, International Rett Syndrome Association (IRSA) Rett Phenotype Database, is a unique international project funded by the IRSA, which brings together child neurologists, geneticists, paediatricians, epidemiologists, researchers and families of affected children. The principal aim of InterRett is to increase the clinical understanding of Rett syndrome throughout the world and with the statistical power of large case numbers, determine any correlations between genotype and the phenotypic characteristics. Since establishment of the database in January 2003, InterRett has registered 286 cases from 24 countries, with family questionnaire data submitted on 242 cases and clinician data on 116 cases. Collated de-identified data gathered from families and clinicians have been incorporated into a searchable online database allowing simple and complex interrogation of the clinical phenotype information. InterRett will also serve as a clearing house for data to encourage inter-country collaboration between researchers and negotiations are in place with several countries for data contributions in excess of 1000 cases. The resulting online database will be an invaluable resource for understanding the nature and management of Rett syndrome, as well as providing a model for other rare childhood disorders.