The clinical characteristics of fulminant type 1 diabetes are abrupt onset of disease, very short (<1 week) duration of diabetic symptoms, ketoacidosis at diagnosis, negativity for islet-related autoantibodies, virtually no C-peptide secretion (fasting plasma C-peptide <0.3 ng/mL), a near normal hemoglobin A1c (HbA1c) level and an elevated serum pancreatic enzyme level. The pathogenesis has not yet been clarified, however the involvement of both genetic background and viral infections has been suggested. We reported a case of fulminant type 1 diabetes. A 37-year-old woman was admitted with stuporous consciousness to our hospital. Four days prior to the admission, she was hospitalized with the diagnosis of acute pancreatitis in another hospital, and at that time her glucose level was 79 mg/dL. Three days after the hospitalization, her state of consciousness became stuporous and she was transferred to our hospital. The laboratory results were as follows: pH 6.94, glucose 1,602 mg/dL, and HbA1c 5.5%. She was negative for islet-related autoantibodies and viral antibodies. HLA haplotypes were DRB1*04:05/*04:06, DQB1*03:02/*04:01 which might be a considerable risk allele for fulminant type 1 diabetes. She was diagnosed with fulminant type 1 diabetes, and has been treated with multiple component insulin regimens.