Rett syndrome is a disorder noted to date only in females and characterized by a pervasive developmental disability following apparently normal early infancy. In addition to gait difficulties, stereotypic hand movements, and loss of communication and purposeful hand skills, autistic-like behavior is an early sign that often results in misdiagnosis. Despite these significant clinical abnormalities, neuropathologic features are modest, and no consistent laboratory abnormality or diagnostic marker has been identified. The current status of research in RS is considered within the context of autism and other disorders in which autistic features may occur, such as the fragile X syndrome. The concept of autism as neurobiologically based behavior is developed. As such, autism is regarded as an umbrella category containing an ever-expanding list of specific disorders.