Positron emission tomography (PET) was performed in four patients with Rett syndrome (RS) to elucidate the cerebral blood flow and oxygen metabolism. Cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), and oxygen extraction fraction (OEF) were measured quantitatively with HEADTOME-IV using 15O2, C15O2 steady state inhalational technique. As to the correlation between CBF and CMRO2, these four patients were compared with nine cases with other neurological disorders. Case 1, the youngest girl among the patients, showed milder symptoms than others. In all patients, no abnormal findings except for mild brain atrophy were seen on CT. In case 1, values of CBF might be lower for her age than normal values by the report of Kety. OEF was markedly reduced in all regions of three patients with RS (cases 2, 3, 4). These decreased OEF might be caused by reduced oxygen metabolism. However, case 1 which was the youngest and had milder symptoms, showed almost normal values of OEF. The distribution of CBF and CMRO2 was thought not to be abnormal in all patients. On the other hand, there was no discrepancy between CBF and CMRO2 in nine neurological control subjects. Decreased value of OEF was known to be seen in mitochondrial encephalopathy as well as an early stage of cerebral infarction. In mitochondrial encephalopathy, OEF was thought to decrease by reduced oxygen metabolism due to disturbance of aerobic glycolysis. Although the cause of reduced oxygen metabolism in RS was obscure, these results suggested that in RS there was some disturbance of oxygen metabolism of the brain, and values of OEF might be related to clinical status of RS or age.