In the Japanese population, the incidence of type 1 diabetes is as low as approximately 2 cases/year/100,000 children, which is much lower compared to that in countries with populations predominantly of Caucasian origin. However, the prevalences of anti-islet autoantibodies in patients with Japanese type 1 diabetes are 60-70% for GAD autoantibodies, 45-50% for insulin autoantibodies (IAA), and 60-65% for IA-2 autoantibodies at disease onset, which are similar to those reported in Caucasian patients. With combinatorial analysis of these autoantibodies, 90% of patients express at least one of these autoantibodies and are classified as type 1A diabetics. There is a significant number of patients with latent autoimmune diabetes in adults (LADA) in Japan, and a high level of GAD autoantibodies has a high predictive value for future insulin deficiency in such patients. Recently, it has been reported that a group of extremely rapid-onset patients presented with diabetic ketoacidosis and a low HbA1c level, called fulminant diabetes mellitus. Although they had severe hyperglycemia, these individuals lacked the expression of anti-islet autoantibodies. With a nationwide survey, it was documented that fulminant type 1 diabetes accounts for approximately 20% of the ketosis-onset patients with type 1 diabetes in Japan. It is currently unknown whether the pathogenesis of fulminant type 1 diabetes is associated with autoimmune response to pancreatic islet beta cells. Japanese patients with type 1 diabetes are clinically heterogeneous, and further investigations are required to clarify the underlying pathogenesis for each subgroup of type 1 diabetes.