The most common wrong diagnosis of children with Rett syndrome has been the infantile autistic syndrome, and it is probable that this false diagnosis continues to be made in many places. This paper is a follow-up to the authors' previous study concerning differential diagnosis between the two syndromes and is based on the observation of 63 patients over the age of 22 months in 10 stimulus situations. In Rett syndrome there were very small individual differences in the sensorimotor performances, which correspond only to the third and fourth stages of Piaget's 'sensorimotor intelligence' scale. There were pronounced individual differences in the levels of basic motor development and, independently from these, in autistic behavioural traits. When the children's behaviour was examined in detail, it became evident that girls with Rett syndrome differ in many subtle ways from those with the autistic syndrome. Behaviour observed in some cases of Rett syndrome but in none of the autistic syndrome included: social (non-autistic) behaviour; no chewing, at most sucking of items of food placed in the mouth; monotonous 'hand-washing' movements with arms flexed in front of chest or chin. In all cases of Rett syndrome but in only some of the autistic syndrome there were: no 'social defence reactions' nor 'primary self-injuring activities'; some form of ataxia and a very restricted repertoire of postures and movements, monotonous in form and speed; and the time spent looking at objects and people was at least as long as the time taken to manipulate objects. Some causes of the characteristic stereotypic hand movements observed in Rett syndrome are also suggested.