Rett syndrome is a severe neurodevelopmental disorder affecting girls, caused by mutations in the MECP2 gene. There are no population-based data on the extent and determinants of health service use in this disorder. The population-based registry, the Australian Rett Syndrome database, was the source of phenotype data. This also contains data from mutation screening and X-inactivation studies. Data on retrospective (n = 152) and prospective (n = 162) health service use were collected in 2000 from a questionnaire and a calendar study, respectively. Health service use was highest in younger cases (P = .001) and lowest in cases with milder phenotypes (P < .001). Random X-inactivation was associated with service use (P = .02). Maternal education, phenotype, and individual mutations were determinants of health service use. The use of a retrospective and prospective data set enabled accurate assessment of service use in Rett syndrome. Both genetic and sociodemographic determinants of health service use were identified, with important implications for the optimal and equitable management of children with Rett syndrome.