Background: We are describing an unusual case of severe hyperglycemia and hypernatremia, resistant to treatment. Case presentation. A thirty year old female with adenocarcinoma of rectum was admitted with increasing lethargy, headache and drowsiness. She deteriorated rapidly and had cardiac arrest, following which she remained comatose. Her initial serum glucose and sodium were normal, but after receiving dexamethasone and mannitol, the serum glucose progressively increased to 54.7 mmol/L and sodium to 175 mmol/L, despite receiving very high dose of intravenous (IV) insulin infusion. She was evaluated for diabetes insipidus because of continued polyuria even after correction of hyperglycemia. Her serum osmolality was 337 mmol/kg, and urine osmolality was 141 mmol/kg which rose to 382 mmol/kg, after receiving 4 mcg of IV Desmopressin. Conclusion: Our patient developed central diabetes insipidus post cardiac arrest and severe dehydration because of diabetes insipidus. Stress of critical illness, dehydration, dexamethasone and IV dextrose infusion were likely responsible for this degree of severe and resistant to treatment hyperglycemia.