Polyuria is defined as an excessive urinary output over a time interval. It is in excess of 4-5ml/kg/hour consistently over a specified period. Polyuria in children is either due to inadequate response to AVP at the collecting tubule or a decreased production of AVP (central diabetes insipidus—CDI) or a normal AVP production and function, but related to solute diuresis or natriuresis. Clinical manifestations depend on the etiology of polyuria. For example, an infant with congenital NDI shows evidence of the disease with polyuria and symptoms of excessive crying and irritability secondary to thirst. The definitive diagnosis can be obtained by inducing hyperosmolality with complete water restriction, thereby stimulating endogenous ADH production and raising the urine osmolality.