Rett syndrome (RTT) is a rare genetic disorder within the autistic spectrum. This study compared socio-demographic, clinical and genetic characteristics of the international database, InterRett, and the population-based Australian Rett syndrome database (ARSD). It also explored the strengths and limitations of InterRett in comparison with other studies. A literature review compared InterRett with RTT population-based and case-based studies of 30 or more cases that investigated genotype and/or phenotype relationships. Questionnaire data were used to determine case status and to investigate the comparability of InterRett and ARSD. Twenty-four case series, five population-based studies and a MECP2 mutation database were identified of which 21 (70%) collected phenotype and genotype data. Only three studies were representative of their underlying case population and many had low numbers. Of 1114 InterRett subjects, 935 born after 1976 could be verified as Rett cases and compared with the 295 ARSD subjects. Although more InterRett families had higher education and occupation levels and their children were marginally less severe, the distribution of MECP2 mutation types was similar.The InterRett can be used with confidence to investigate genotype phenotype associations and clinical variation in RTT and provides an exemplary international model for other rare disorders.