Rett syndrome (RTT) is an X-linked severe neurodevelopmental disorder. Despite their good appetite, many females with RTT meet the criteria for moderate to severe malnutrition. The pathological mechanism is barely understood. Although feeding difficulties may play a part in this, other constitutional factors as altered metabolic processes are suspected. Irregular breathing is a common clinical feature, reflecting the immaturity of the brainstem in RTT. The primary pathophysiology is a defective control mechanism of carbon dioxide exhalation that leads to chronic respiratory alkalosis or acidosis. We assume that chronic respiratory acidosis or alkalosis causes derangement of the metabolic equilibrium in RTT females with important nutritional consequences.