Fulminant type 1 diabetes mellitus (FT1D) is a novel subtype of type 1 diabetes. Given below are the specific clinical manifestations shown at the onset of FT1D: 1) Hyperglycemia with diabetic ketoacidosis 2) Plasma glucose level is more than 16.0 mmol/L (288 mg/dL), whereas glycated hemoglobin level is less than 8.7% 3) Urinary C-peptide excretion