Rett's syndrome is a progressive disorder in female patients, characterized by autistic behavior, dementia, ataxia, loss of purposeful use of the hands, and seizures. The electroencephalographic (EEG) characteristics of 17 patients with Rett's syndrome, studied between the ages of 1 and 16 years, are reported and correlated with a recently proposed system of clinical staging. Although a specific diagnostic EEG pattern was not seen in these patients, we did observe a progressive deterioration in the EEG, characterized by a slowing of EEG activity, a loss of normal sleep EEG characteristics, and the appearance of multifocal epileptiform abnormalities, followed by a pattern of generalized slow spike-wave activity. These characteristics appear to be typically seen in patients with Rett's syndrome and can be correlated with clinical staging. The EEG may be of benefit in identifying variations or subgroups in patients with Rett's syndrome as a complement to the clinical examination. © 1987, American Medical Association. All rights reserved.