Diabetes insipidus is a rare endocrine disorder in paediatric patients. Polyuria is a cardinal manifestation that is extremely difficult to recognize in diapered infants. Careful urine quantification is the key to diagnosis in appropriate clinical setting. We report a case of a 4 months old infant presenting with an acute life threatening event following an episode of vomiting and decreased oral intake. She had profound hypernatremia which persisted after stabilization. Polyuria unrecognized by the mother was revealed by 24-hour urine output measurement. A diagnosis of diabetes insipidus was made after appropriate laboratory investigations including serum and urine osmolality. The central nature of the disease was confirmed by neuroimaging which showed holoprosencephaly.