Eleven female patients with Rett syndrome were evaluated for detecting the pathogenesis. Clinical symptoms were characterized by their orderly sequence of occurrence of particular symptoms at particular ages starting from early infancy, and their progression. Increment of head circumference tapered from late infancy, resulting in microcephalus which corresponded with the clinical features. Surface EMG revealed the tremulous rhythmic discharge underlying the characteristic stereotyped movement of the hands. Serial polysomnographical examinations showed abnormalities of the tonic and phasic components of sleep and increment of % REM stage with age. The results of these clinical, laboratory and polysomnographical examinations were discussed, comparing with other neurological diseases and knowledge of animal experiments. From these findings the pathophysiology of Rett syndrome could be explained by the early and progressive lesions in the brain stem nuclei, which influence the maturation and function of particular parts of the higher central nervous system. Serotonergic and catecholaminergic neurons might have important roles in the pathophysiology of this syndrome. However, biochemical and histochemical examinations of the brain are necessary for detecting the pathogenesis and etiology. And the cause of gynecopathy also remains to be clarified.