We report an 18-year-old male with a history of polyuria, polydipsia, and thirst since childhood. In a hypertonic saline infusion test, the patient's plasma vasopressin rose only to 0.28 pg/ml. In a water deprivation test, his urinary osmolality rose only to 189 mosmol/kg and then rose to 538 mosmol/kg by vasopressin administration. A T1-weighted magnetic resonance imaging (MRI) scan revealed a loss of the posterior pituitary bright spot. Sequencing of the vasopressin gene showed a heterozygous point mutation that replaced Ala at the carboxyterminus of the signal peptide with Thr. His father also had similar history, and we therefore diagnosed his illness as familial central diabetes insipidus.