AbstractWe have studied 23 patients with Rett syndrome with particular reference to the character and natural history of the clinical disorder.We found a prevalence of 0.8 per 10,000 girls 0–14 years in the region from which cases came. We consider that the disorder of tone, posture and movement is extra‐pyramidal in nature and suggest that the gradual emergence of its fully developed pattern leads to the familiar regression in skills on presentation. We have not found proof of dementia at this stage but rather of severe mental handi