Severe hyponatremia (serum sodium levels less than 120 mmol/l) is associated with increased morbidity and mortality. We report three cases where patients developed hyponatremia and severe neurological manifestations after medical treatment. The first two patients experienced episodes of general seizures and coma, but recovered in 24 hours without neurologic sequelae after correction of the electrolyte disturbance. The third patient developed the syndrome central pontine myelinolysis with pseudobulbar palsy and quadriparesis. Marked improvement occurred, however, and in three months the patient was almost completely recovered. The development of hyponatremia deserves special attention in connection with the use of diuretics, infusion of sodium-free carbohydrates and immediately after operation.