We report a 65-year-old woman with paraneoplastic cerebellar degeneration (PCD) who showed reduced cerebellar metabolism with preserved blood flow. She was admitted to Gunma University Hospital because of progressive gait and speech disturbances. Neurologic examination revealed nystagmus, dysphagia, explosive speech, reduced muscle tone in limbs, and marked truncal and limb ataxia, and mild hypesthesia in hands and feet. Cranial MRI demonstrated slight cerebellar atrophy. Laboratory findings disclosed high levels of serum CA19-9 and other tumor markers, and positive anti-Yo antibody, indicating that she had PCD. A specimen obtained from an axillary lymph node revealed metastasis of poorly differentiated adenocarcinoma, although systemic and vigorous checkup failed to find its origin. Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured using positron emission tomography (PET) 15 months after the onset. CMRO2 was clearly decreased in the cerebellum, while CBF was almost normal. Moreover, PET with 2 18F-fluoro-2-deoxy-D-glucose (FDG) revealed that glucose metabolism was also reduced in the cerebellum. Single photon emission tomography using 99mTc-ethyl cysteinate dimer (ECD) showed a normal blood flow pattern in the whole brain. These results indicated that uncoupling of circulation and metabolism in the cerebellum of this patient. There are several reports showing uncoupling of cerebral perfusion and metabolism in ischemic disorders, encephalitis, mitochondrial diseases, brain tumors, epilepsy and Gaucher disease, although its pathophysiology is not elucidated. Because anti-Yo antibody evidently gives a suppressive influence on the cerebellar neurons, understanding the way the autoantibody acts may give a clue to the mechanism of reduced cerebellar metabolism with preserved perfusion in PCD.