Rett syndrome is a unique disorder mainly in girls characterized by global deceleration of psychomotor development and subsequent loss of acquired cognitive and motor skills, occurring after 6 to 18 months of apparently normal development. Previous study described that 26% of the cause of their death was sudden and unexpected. Although the cause of sudden death is still unknown, respiratory disorder, cardiac arrhythmia, and severe seizure might be related to sudden death. The possibility of sudden death should be informed to families of patients with Rett syndrome. Electrocardiogram should be examined in older patients, because prolongation of QTc and T wave abnormalities increase with advancing stage.