Four girls affected with the Rett syndrome showed stereotyped hand movements and psychomotor regression beginning in the first or second year of life after initially normal development, with total loss of language, a severe gait, and chewing and swallowing disturbances. Breathing during active wakefulness was abnormal, with clusters of arrhythmic, tachypneic and irregular respiratory acts, intermixed with prolonged apneic episodes causing cyanosis and even fainting. On the other hand, breathing was regular during both NREM and REM sleep stages. Breathing impairment in the Rett syndrome is characterized by an inability to maintain normal respiratory patterns during wakefulness, and represents a functional disturbance of the behavioral control of breathing, which occurs during active wakefulness.