A brief review of the Rett syndrome is presented. The main clinical features are onset of mental deterioration, in girls only, between the ages of6 and 18 months, all of whom later develop peculiar stereotyped repetitive movements resembling hand washing. The cause is unknown; the condition occurs more often than phenylketonuria and laboratory tests are normal. Random sequences of hyper-ventilation-apnoea-normal breathing appear in a considerable number of girls and are accompanied by certain EEG features. The nature of the repetitive manual and respiratory phenomena is discussed. We suggest that these behavioural peculiarities may be a form of communication, possibly pleasurable and to some extent interchangeable.