It has been reported that fulminant type 1 diabetes is a novel subtype of type 1B diabetes. However, whether the etiology of fulminant type 1 diabetes is associated with an autoimmune or nonautoimmune process remains to be solved. In order to further characterize fulminant type 1 diabetes, we compared the clinical, immunological and genetic characteristics with those of acute-onset type 1A diabetes. Nine patients with fulminant diabetes and nine patients with acute-onset type 1A diabetes, who had been newly diagnosed during 1998-2001, were analyzed. In female patients of child-bearing age, the onset of diabetes occurred during pregnancy or after delivery in three cases of six fulminant cases, but not in any of seven type 1A diabetes. Eight of nine fulminant patients had fever immediately prior to the onset of hyperglycemic symptoms, whereas only one of nine type 1A patients had this (P=0.002). In Japanese type 1 susceptible HLA haplotypes, DRB1*0901-DQB1*0303 was more frequent in type 1A diabetes than fulminant diabetes (7/18 vs. 0/18, P=0.004), whereas the frequency of DRB1*0405-DQB1*0401 was similar (type 1A 4/18 vs. fulminant 6/18). Therefore, pregnancy, possible viral infection, or HLADRB1*0405-DQB1*0401 may contribute to the onset of fulminant type 1 diabetes.