Efforts to understand and prevent pontine and extrapontine myelinolysis have focused on the correction of hyponatremia, but controversy persists. We report a woman who presented in hyperosmolar diabetic coma with hypernatremia (169 mEq/l) and hyperglycemia (954 mg/dl). Plasma sodium rapidly increased to 188 mEq/l before gradually returning to normal. She remained obtunded and died 21 days later. Autopsy showed widespread, symmetrical demyelination involving the subcortical white matter, corpus callosum, anterior commissure, extreme, external, and internal capsules, fornix, thalamus, cerebellum, and lateral pons. The central pons and lateral geniculate nuclei were uninvolved. This case illustrates that lateral pontine and extrapontine myelinolysis can be associated with hypernatremia and hyperosmolality. In both hypo- and hypernatremic states, the significant event may be an increase in serum sodium or serum osmolality of sufficient rapidity and magnitude.