A 41 year old lady was referred to the endocrine clinic by her respiratory physician. She is known to have Langerhans cell histiocytosis (LCH) with lung involvement. She is having yearly follow up under respiratory team and not needing any treatment for LCH. In endocrine clinic she confirmed that she is drinking around 9 to 10 L of water / day and having quite significant polyuria. She works in Tesco. She does not have any other medical condition and not taking any regular medication. There is no history of change in body weight, lethargy, oral or genital thrush, cough or abdominal pain. Her Hba1c was 37 and Na 148 with normal eGFR. Her early morning cortisol 430. She had a water deprivation test which confirmed central diabetes insipidus (CDI). Her MRI pituitary showed hypophysitis. CT Neck, thorax, abdomen and pelvis: was unremarkable. Other autoimmune, infiltrative and infections were ruled out. Her rest of pituitary profile was checked and it was normal. A clinical diagnosis of central diabetes insipidus secondary to LCH was made. She was started on desmopressin nasal spray. Symptoms dramatically improved with desmopressin nasal spray 5 mg in the morning and 10 mg at night. Appropriate follow up arranged in endocrine clinic to monitor her electrolytes. LCH is a rare and probably underdiagnosed disease with a wide range of presentation. Due to infiltration of the hypothalamic pituitary axis, CDI can even be the first manifestation, even before LCH is diagnosed. Therefore, LCH should be considered in the diagnostic workup of CDI.