We report a 63 year-old woman who developed central pontine and extrapontine myelinolysis after rapid correction of hyponatremia. Lesions on brain MRI showed hypermetabolism on ¹⁸FDG-PET scan in the early stage of the disease and became hypometabolic on the follow-up scan. We suggest that active microglia and astrocytes are the main cause of the increased glucose metabolism.