Abstract Cerebral and pontine myelinolysis were found in 2 patients, in 1 associated with inappropriate secretion of ADH and low serum omolality and in the other with diabetic coma. There was no clinical or autopsy evidence of alcoholism or malnutrition in either case. In the first patient the myelinolysis mainly affected smaller bundles of myelinated fibres, while in the diabetic the lesions were in the centre of larger masses of white matter. It is suggested that the myelinolysis was caused by cellular osmotic and metabolic disturbances which were secondary to a combination of factors, the most important being prolonged, fluctuating abnormalities of the serum sodium in the first patient and of the blood glucose in the second. In both cases during the early phases of therapy there was further deterioration with disturbances of respiration and hypotensive episodes which could have increased the effect of the fluid and electrolyte imbalance.