The prevalence of diabetes mellitus(DM) is increasing in Korea and is estimated at 7~8%. This high prevalence is most likely the result of a complex interaction between genetic and environmental factors. The lifestyle and diet of the Korean population have changed significantly during the last several decades. In general, the Koreans have become more sedentary and they consume westernized diet. In addition to the increase of prevalence, the clinical characteristics of DM in Korea are somewhat different from those in Western countries. The incidence rate of type 1 DM in Korea is one of the lowest reported in the world. There are many patients with atypical type 1 or type 2 DM, initially classified into type 2 DM but requiring insulin eventually. There are many patients with non-obese type 2 DM, and many of them lose weight significantly during the course of disease. In addition, some patients with type 2 DM have autoantibodies to the beta cell-specific antigen. The defect in insulin secretion is prominent in non-obese subjects with impaired glucose tolerance or early phase of type 2 DM. The Korean patients with type 2 DM have a markedly decreased beta cell mass. The impaired insulin secretion may play a role in the pathogenesis of non-obese type 2 DM in Korea. Of note, similar features are also observed in Japanese patients. Unfortunately, there are few studies that evaluated the pathogenesis and clinical characteristics of Korean patients with DM, particularly in a cohort-based prospective manner.