BACKGROUND: Korean type 2 diabetic patients are known to differ from western diabetes because of their unique characteristics, such as non-obese but centrally obese anthropometry and relatively more insulin secretory defects than insulin resistance compared to western diabetic patients. METHODS: We recruited 1,646 diabetic patients in the present study and excluded the 45 patients with fasting C-peptide < 0.20 nmol/L. We had assessed insulin secretion by fasting serum C-peptide level and insulin resistance by short insulin tolerance test (Kitt ; rate constant for plasma glucose disappearance, %/min) in the private diabetes clinic. The insulin secretory defect was divided by severe (C-peptide < 0.37 nmol/L), moderate (C-peptide 0.37~0.56 nmol/L), and normal (C-peptide > or = 0.57 nmol/L) group. The insulin resistance was divided by insulin resistant (IR) (Kitt < 2.5 %/min) and insulin sensitive (IS) (Kitt > or = 2.5 %/min) group. RESULTS: We analysed the data of 1,601 type 2 diabetic patients (831 men and 770 women, age 56.5 +/- 10.8 years, duration of diabetes 9.6 +/- 7.3 years). The prevalence of BMI > or = 25.0 kg/m2 is 42.5% and BMI > or = 23.0 kg/m2 is 70.2%. The prevalence of abdominal obesity (waist > or = 90 cm in men and 80 cm in women) is 45.2% (36.0% and 55.2%, respectively in men and women). Fasting C-peptide level is 0.64 +/- 0.29 nmol/L and Kitt value is 2.03 +/- 0.96%/min. According to fasting C-peptide level, the degree of insulin secretory defect were severe (13.1%), moderate (33.0%) and normal (53.9%). According to Kitt value, the IR group is 70.6% and the IS group is 29.4%. CONCLUSION: Obese type 2 diabetes is markedly increasing in Korea. Therefore, the major problem in Korean type 2 diabetic patients is being changed into insulin resistance instead of insulin secretory defect.