Forty girls with Rett syndrome were included in a study of behaviour, with particular regard to the hands, before, during and after regression. Data was taken from examination of each girl, and in some cases from detailed developmental histories given by parents, and films taken before, during and after regression. The important findings are shown in two tables and described. Pre-regression abnormalities included hypotonia, jerky incoordination, an excess of patting or waving activity and involuntary movements which included alternate opening and closing of the fingers and twisting of wrists or arms. Hand use did not progress beyond the ten- to twelve-month stage. Language did not develop beyond the stage of one word utterances. When well-developed, the stereotyped hand movements were simple and clumsy, consisting of tapping, rubbing and clasping, with the hand moved as a unit. Before regression hands were usually separate, during regression usually together and thereafter with increasing age inclined to separate again. Voluntary hand use was observed when girls were relaxed and strongly motivated, particularly during musical interactions. The characteristic abnormalities of behaviour in pre-regression Rett syndrome, and hand behaviour in later childhood should allow earlier and more accurate diagnosis.