Fulminant type 1 diabetes is a novel subtype characterized by a markedly rapid progression and almost complete destruction of pancreatic beta cells. The number of patients in Japan has been speculated to be 5,000-7,000. A marked decrease of beta cells in addition to alpha cells and mononuclear cell infiltration both in the endocrine and exocrine pancreas are characteristic pathological findings in recent-onset fulminant type 1 diabetes. Laboratory examinations have revealed a high blood glucose level, near normal hemoglobin A1c, ketosis or ketoacidosis, elevation of serum pancreatic exocrine enzymes, and absence of anti-islet autoantibodies such as anti-glutamic acid decarboxylase (GAD) antibody or anti-insulinoma-associated antigen-2 (IA-2) antibody at disease onset. Genetic factors of HLA-DR-DQ, CTLA-4, and HLA-B are associated with this subtype. Both diagnostic criteria for screening and establishing have been announced by the Japan Diabetes Society. In approximately half of fulminant type 1 diabetes, HbA1c was lower than 6.2% at disease onset, indicating that newly proposed diagnostic criteria of diabetes (HbA1c > or = 6.5%) from the joint committee of the American Diabetes Society, the European Association for the Study of Diabetes, and the International Diabetes Federation are not applicable to fulminant type 1 diabetes. In conclusion, all medical practitioners must remember that fulminant type 1 diabetes, an extremely rapidly progressing type of diabetes, does exist, and must pay special attention to avoid overlooking this disease.