Polyuria is an uncommon clinical presentation in paediatric practice. When diabetes mellitus has been excluded by history taking and preliminary investigations and impaired renal concentrating ability is confirmed, water deprivation test (WDT) is necessary to differentiate among central diabetes insipidus (CDI), nephrogenic diabetes insipidus, or primary polydipsia. Traditionally, responsiveness to desmopressin injection is defined as urine osmolality >750 mOsm/kg. However, that level could not be reached in the review of our patients. We discuss how to arrive at the diagnosis of CDI in WDT. An approach to polyuria and WDT will also be discussed.