[The Rett syndrome].

A 12-year-old girl with Rett's syndrome is presented. This syndrome, exclusively reported in girls, is characterized by a normal psychomotor development up to the age of 6 to 18 months, followed by developmental stagnation and then rapid progressive dementia, loss of purposeful use of the hands replaced by stereotypic hand movements, truncal ataxia, spasticity, seizures and microcephaly. The syndrome is not rare; prevalence has been reported as 0.65/10 000 girls.