Introduction: Adipsic hypernatremia is characterized by chronic and recurrent episodes of severe hypernatremia associated with a lack of thirst and often associated abnormal development of the midline structures of the proencephalon, including corpus callosum, septum pellucidum. We report a rare case of adipsic hypernatremia associated with congenital defect of the corpus callosum and mental retardation. Case report: We report a 31-year-old male patient presenting with asymptomatic hypernatremia. He had been previously diagnosed with inguinal hernia. He was visited to our hospital for inguinal hernia operation. Laboratory values were as follows: serum Na, 166 mEq/L; blood urea nitrogen(BUN) 20.1 mg/dL; creatinin(Cr) 1.39 mg/dL; Physical examination revealed an alert mental status with mental retardation. The patient was reffered to division of nephrology for evaluation of hypernatremia. On admission, serum Na 167 mEq/L, blood urea nitrogen(BUN) 23 mg/dL, creatinin(Cr) 1.36 mg/dL, serum osmolality 347mOsm/Kg, urine osmolarity 869mOsm/kg. but he did not report thirst, weakness, polyuria. His vital sign was stable including pulse pressure. On physical examination, body weight, height and sexual development were normal. A MRI scan of the brain demonstrated anomalous corpus callosum and non-visualized septum pellucidum. And sex hormone, combined pituitary function test were normal. A water deprivation test and water load test were comducted. Plasma arginine vasopressin (AVP) level was low, and neither stimulated by hyperosmolality nor suppressed by hypoosmolality. He was diagnosed a hypothalamic adipsic hypernatremia associated with corpus callosum defect. He was educated about the importance of water intake and we got him to drink more water. 1 month later, the follow-up serum Na, osmolality decreased gradually to 137mEq/L, 281mOsm/Kg. Conclusion: we report an unusual case that showed hypothalamic adipsic hypernatremia, congenital defect of corpus callosum and mental retardation. This case suggests hypothalamic adipsic hypernatremia must be suspected when a dehydrated patient denies thirst and that there is a need to hormone study and forced hydration in these patients including brain images.