The syndrome which Andreas Rett and Bengt Hagberg first brought to our attention has been observed for eighteen years. It appears to be unlike any other disease of childhood. The original descriptions of Rett syndrome, as being autistic, degenerative and progressive are now, based on longitudinal observations, being questioned. In this paper the morphologic changes in the nervous system in Rett syndrome are summarized. The evidence against a degenerative process are presented, and the alterations are considered, along with the clinical features in terms of a possible arrest of brain development.