Hyponatremia is an important and common electrolyte abnormality. It is defined as serum sodium level <135 mmol/l. Hyponatraemia can be classified on the basis of serum sodium concentration, rate of development, symptom severity, seerum osmolality, and volume status. The classification is relevant for patient management. Symptoms can be moderate to severe like vomiting, cardio-respiratory distress, somnolence, seizures and coma. Overt neurologic symptoms are most often due to very low serum sodium levels, resulting in intracerebral osmotic fluid shifts and brain oedema. Treatment has to balance between possible complications like brain oedema and osmotic demyelination syndrome. In this case the speed of correction is very important. Limiting the increase in seerum sodium concentration to a total of 10 mmol/l during the first 24 h and an additional 8 mmol/l during every 24 h thereafter until the concentration reaches 130 mmol/l has been recommended. A case of osmotic demyelination syndrome is presented.