Rett syndrome causes severe autonomic dysregulation, probably due to brainstem dysfunction. Because the brainstem plays a decisive role in cardiorespiratory regulation during sleep, we investigated cardiorespiratory function in 12 girls with Rett syndrome, day and night, for 1 week in their home environment. Heart rate and breathing were recorded via standard three-lead electrocardiogram. Depth and frequency of respiratory movements were measured via changes in impedance. All children were scored clinically, and the association with cardiorespiratory function was examined. The total recording time for all patients was 1114 hours (535 during wakefulness; 579 during sleep), and 77 +/- 22 hours (median +/- standard error of the mean) per individual. All subjects manifested apnea, shallow breathing, or hypoventilation, when awake and during sleep. A majority had bradycardia or tachycardia. The frequencies of respiratory and heart alarms were similar during wakefulness and sleep. Bradycardia events predominated during sleep. The only significant correlation between clinical score and cardiorespiratory regulation was found for muscular-skeletal function and breathing abnormalities during wakefulness. We conclude that Rett syndrome is characterized by disturbed breathing and heart rate during sleep. The severity of cardiorespiratory dysfunction exhibited marked intra- and interindividual differences.