Fulminant Type 1 diabetes is a novel subtype of Type 1 diabetes. In this disease, extremely rapid and almost complete beta-cell destruction occurs, resulting in nearly no residual insulin secretion even just after the onset. The number of patients presumably amounts to 5,000-7,000 in Japan. The involvement of both, genetic background and viral infection, has been suggested in the pathogenesis of this disease. Diagnostic criteria were established by a committee of the Japan Diabetes Society in 2004. Intensive insulin therapy is a standard therapy. We should pay special attention to early development of microvascular complications in fulminant Type 1 diabetes.