Two patients with central pontine myelinolysis are described. Both were middle aged women presenting with a history of protracted vomiting and drowsiness. Hyponatraemia (serum sodium 96 to 100 mmol/L) was a feature in both patients. No underlying malignancy, alcoholism, malnutrition or other serious disease was identified. Correction of electrolyte abnormalities was accompanied by deterioration in level of consciousness and development of a neurological syndrome characterized by quadriparesis, dysphasia and mutism. Death followed and histopathological examination confirmed classical myelinolysis in the central pons and extensive similar, though not identical, lesions in the cerebral hemispheres in both cases. The pathophysiological basis of the lesions is likely to be a special metabolic susceptibility of oligodendroglial cells in areas where neurones, glial cells and myelin sheaths lie in close proximity to one another.