BACKGROUND: Recently, the number of young diabetic patients is increasing. It is important to understand the characteristics of young diabetes and classify it correctly to manage these patients successfully. We aimed to classify young onset diabetes according to etiology and evaluate the clinical characteristics. METHODS: Young patients (15~30 years old) who have been treated diabetes in Kyunghee medical center in 2004 were included. We investigated family history of diabetes, disease duration, body mass index (BMI), the history of diabetic ketoacidosis, HbA1c, fasting C-peptide, autoantibody, lipid profile and treatment method via medical records. RESULT: Total 85 patients (M:F 40:45) were evaluated. Type 1 diabetes was 45.9%, type 2 diabetes was 23.5% and unclassified group was 25.9%. Many type 2 diabetic patients were overweight or obese (94.8%). Most young diabetic patients were using insulin (95.4%). Many type 1 diabetic patients have been treated by insulin only and many type 2 diabetic patients have been received combined therapy of insulin and oral hypoglycemic agent. The recent HbA1c was average 8.32 +/- 2.7%. The prevalence of diabetic retinopathy, neuropathy and nephropathy was 32.9%, 22.4% and 16.4% as each. CONCLUSION: Nearly half of young onset diabetes was type 1 diabetes but many others were also classified to type 2 diabetes or unclassified group. It is important to provide a consistent algorithm for assessment and investigation for newly diagnosed young diabetic patients. More education and effort are required to control diabetes strictly and prevent its complication.