OBJECTIVE: To describe the clinical and immunologic characteristics of fulminant type 1 diabetes, a novel subtype of type 1 diabetes, we conducted a nationwide survey. RESEARCH DESIGN AND METHODS: History and laboratory data, including islet-related autoantibodies, were examined in 222 patients with fulminant and nonfulminant type 1 diabetes in our hospitals in addition to another 118 patients with fulminant type 1 diabetes located outside our hospitals in Japan. RESULTS: In our hospitals, of the 222 patients studied, 43 (19.4%) were diagnosed with fulminant type 1 diabetes, 137 (61.7%) were classified as having autoimmune type 1 diabetes, and 42 were type 1 diabetic subjects who were not fulminant and did not have anti-islet antibodies. An additional 118 fulminant patients outside our hospitals were enrolled, making a total of 161 fulminant type 1 diabetic subjects (83 male and 78 female subjects; 14 children/adolescents and 147 adults) identified from all over Japan. (In 2000, the average incidence was three cases per month.) Flu-like symptoms and pregnancy were more frequently observed in the fulminant than in the autoimmune group (P < 0.001). In the fulminant patients, 4.8% were positive for anti-GAD antibodies and none were positive for anti-islet antigen 2 antibodies. CONCLUSIONS: Fulminant type 1 diabetes is a distinct subtype and accounts for approximately 20% of the ketosis-onset type 1 diabetes cases in Japan. Flu-like symptoms are characteristic of disease onset. Metabolic derangement is more severe in this subtype than in autoimmune type 1 diabetes.