Contents Severe hypernatremia, values above 180 mEq, is associated with a high mortality rate, particularly in adults. There are few reports about severe hypernatremia in patients with diabetic ketoacidosis (DKA). Hypernatremic myopathy have been reported with severe dehydration, acute kidney injury and rhabdomyolysis. In diabetic patient with mental retardation, dehydration can be caused by osmotic diuresis and exacerbated by hypodipsia. Therefore, intense education of regular oral hydration seems to be important. We report a 17-year-old patient with reversible severe hypernatremia (serum corrected sodium 180 mEq/L) with primary hypodipsia and without DKA. The patient visit emergency room due to agitation and weaknesses on both lower extremities. He had mental retardation, dysgenesis of corpus callosum, complete cleft palate. He showed 46XY normal karyotype in chromosomal study. Initial laboratory findings included that hyperglycemia (serum glucose 695 mg/dL), acute kidney injury (serum creatinine 1.86 mg/dL, estimated GFR 50.54 ml/min), acute hepatitis (serum AST 61 IU/L, ALT 117 IU/L), myopathy (serum CK 762 IU/L), high serum and high urine osmolarity (serum 420 mOs/kg, urine 899 mOs/kg). After hydration with quarter-isotonic saline and intensified insulin therapy, his symptoms and abnormal laboratory findings improved completely. And both leg weaknesses were also recovered.