Acquired central diabetes insipidus (CDI) often occurs abruptly after a cranial event causing hypothalamic or pituitary damage. We present a case of a patient with pre-existing and clinically unapparent CDI which was unmasked after renal transplantation. A 60 year old woman with end-stage renal failure due to autosomal dominant polycystic kidney disease (ADPKD) underwent renal transplantation. She was noted to be markedly polyuric and polydipsic after the transplant. A fluid deprivation test was unequivocally positive for CDI, and desmopressin treatment resulted in immediate symptom relief. Neuroimaging revealed a midline defect in the region of the hypothalamus. She had a history of an intracerebral aneurysm that had ruptured, requiring extensive neurosurgery many years previously. This case demonstrates a rare instance of pre-existing but clinically unapparent CDI unmasked by renal transplantation. It is likely that renal failure due to ADPKD disguised her CDI prior to transplantation. A previous intracerebral insult from an aneurysmal bleed is the likely cause of her vasopressin deficiency.