This report deals with an elderly lady with Guillain-Barre Syndrome (GBS), who presented with features of unusually severe hyponatraemia. The hyponatraemia was probably due to the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The hyponatraemia resolved with water restriction and infusion of hypertonic saline; GBS was treated with human immunoglobulin (IVIG). This patient's experience stresses the importance of monitoring serum sodium levels as hyponatraemia has been identified to be a marker of poor prognosis in GBS.