A 23-year-old woman was referred to the Endocrine Clinic complaining of increasing thirst and passing large volumes of urine. She was drinking up to three 2 L ottles of water each day, in addition to tea and coffee. Over the previous 6 months she had started waking at night needing to pass urine and to drink. There was no history of headache, visual impairment, or psychiatric disturbance, no history to suggest pituitary dysfunction and no family history of note. She was taking no medication Blood tests showed normal glucose, potassium and calcium levels. Further investigations showed her to have 24-hour urine volume of 4.3 L and serum osmolality of 302 Osmol/kg with a simultaneous urine osmolality of 276 mOsmol/kg. During a formal water deprivation test, serum osmolality rose with an impaired response of urine osmolality. After intramuscular administration of des-amino-des-aspartate-arginine vasopressin (DDAVP, a long – acting analogue of antidiuretic hormone) her urine promptly concentrated, confi rming a diagnosis of cranial diabetes insipidus (DI). She was initially treated with intranasal DDAVP, subsequently converting to oral therapy. Endocrine and radiological investigations of the hypothalamus and pituitary revealed no evidence of a space occupying lesion.