Hyperglycemia and hyperosmolar hyperglycemic syndrome (HHS) are the rare adverse effects of diazoxide. Furosemide has been reported to worsen glucose tolerance and cause HHS. A 5-yr-old girl presented to the emergency department with complaints of tachycardia, polyuria, and lethargy for 1 wk prior to hospitalization. She was treated with two diuretics for aortic valve reflux disease and diazoxide for congenital hyperinsulinemia. She was diagnosed with HHS based on her serum glucose level of 529 mg/dL and serum osmotic pressure of 357 mOsm/kg. There were no findings suggestive of new-onset diabetes mellitus. She had fever on admission and, was diagnosed with a urinary tract infection. The blood diazoxide level at the time of hospitalization was 25 µg/dL. Diazoxide use, even in patients with low diazoxide levels, may cause hyperglycemia. Patients on diuretics and diazoxide must be carefully monitored, considering the risk of developing HHS.