Neurologic disorders developing after correction of severe, symptomatic hyponatremia were studied in 14 patients. None had a hypoxic event or other identifiable cause for the neurologic illness. Neurologic deterioration began about 3 days after correction and often followed a period of improvement in hyponatremic encephalopathy. Although the symptoms were as mild as transient confusion in 1 patient, they were more severe in the others. Typically, spastic quadriparesis, pseudobulbar palsy, and impairment in the level of consciousness progressed for up to 7 days. Improvement generally began 2 weeks after correction and continued for up to a year in some patients. Routine spinal fluid analysis was usually normal, but myelin basic protein concentration was elevated in all patients in whom it was measured. Electroencephalograms commonly showed nonfocal slowing. Brainstem auditory evoked potential latencies were prolonged in some patients. Brain imaging was normal in the initial week of illness, while later scans, obtained in 9 patients, showed central pontine and/or symmetric extrapontine lesions. The clinical manifestations and distribution of lesions seen on imaging demonstrate that neurologic illness following correction of hyponatremia is due to myelinolysis. Although this neurologic disorder typically followed an elevation in serum sodium > 18 mEq/L/24 hr, it sometimes followed a rise as slow as 10 mEq/L/24 hr and 21 mEq/L/48 hr. Whenever possible, the rate of correction of hyponatremia should be kept below these values in order to minimize the risk of myelinolysis.