Rett syndrome, a peculiar neurodevelopmental deficiency affecting females, which starts in early childhood, is reviewed based on a Swedish series of 170 females, 2-52 years of age (to December 1994). To date, the well recognized classical phenotype was found in 75% of cases. Atypical variant forms, mainly more mildly affected mentally retarded girls and adolescent women, were still in a minority, but constitute, with increasing experience, an expanding cohort. The biology and genetics of the condition seem puzzling. Traditional neurodegenerative pathology has been excluded. An age-limited neurodevelopmental, as yet unknown, brain growth deficiency, is at present indicated. The syndrome is most probably genetically determined, but the mode of transmission is not convincingly compatible with any known pattern.