We report a rare case of the concurrent manifestation of central diabetes insipidus (CDI) and type 2 diabetes mellitus (DM). A 56 year-old man was diagnosed as a type 2 DM on the basis of hyperglycemia with polyuria and polydipsia at a local clinic two months ago and started an oral hypoglycemic medication, but resulted in no symptomatic improvement at all. Upon admission to the university hospital, the patient`s initial fasting blood sugar level was 140mg/dL, and he showed polydipsic and polyuric conditions more than 8 L urine/day. Despite the hyperglycemia controlled with metformin and diet, his symptoms persisted. Further investigations including water deprivation test confirmed the coexisting CDI of unknown origin, and the patient`s symptoms including an intense thirst were markedly improved by desmopressin nasal spray (10 μg/day). The possibility of a common origin of CDI and type 2 DM is raised in a review of the few relevant adult cases in the literature.