Fulminant type 1 diabetes is a recently discovered subtype of type 1 diabetes. It is defined as diabetes in which the process of beta-cell destruction and the progression of hyperglycemia and ketoacidosis are extremely rapid. The pathogenesis of this disease remains to be clarified, but the involvement of both genetic background-especially human leukocyte antigen genes-and viruses has been suggested. Fulminant type 1 diabetes has the following clinical characteristics: duration of hyperglycemic symptoms is 4 days on average; there is a high prevalence of preceding common-cold-like and gastrointestinal symptoms; there is a near-normal level of glycated hemoglobin in spite of very high plasma glucose levels associated with ketoacidosis; the disease is sometimes related to pregnancy; and there are increased serum pancreatic enzyme levels, absent C-peptide levels, but virtually no detectable autoantibodies against constituents of pancreatic beta cells. The presence of the above characteristics strongly indicates the diagnosis of fulminant type 1 diabetes. Once the diagnosis of this disease is suspected, treatment of diabetic ketoacidosis must be started immediately, as in all other cases of type 1 diabetes. Otherwise, the death of the patient is likely to occur within 24 h. All medical practitioners must remember that this extremely rapidly progressing type of diabetes does exist, and they must pay special attention not to overlook it.