BACKGROUND The aim of this study is to clarify the clinical characteristics of non-obese, adult-onset diabetes requiring insulin treatment and to compare the different characteristics of the three groups categorized according to diabetes classification. METHODS: Total 128 diabetic patients who were non-obese (BMI < 25kg/m2) and had been diagnosed with diabetes after 20 years old, requiring insulin treatment were enrolled in the study. We divided the patients into three groups : 56 patients with type 1, 37 with unclassifiable, and 35 with type 2 diabetes. The type of diabetes was assigned by comparing serum C-peptide concentration and clinical phenotypes. RESULTS: Type 2 and unclassifiable diabetes had no differences in BMI, the interval to use insulin, daily insulin dose, the level of HDL cholesterol and the positive rate for GAD Ab, but type 1 diabetes didn't. However, type 1 diabetes and unclassifiable group was lower prevalence of microvascular complications than type 2 diabetes (retinopathy 38.2, 52.8, 84.8 % ; nephropathy 37.7, 36.7, 74.2 % ; neuropathy 36.7, 36.7, 72.7 %, P<0.05). The prevalence of macrovascular complications was higher in the order of type 1, unclassifiable, and type 2 diabetes (11.1, 29.4, 72.7 %, respectively, all P<0.05). CONCLUSION: The clinical characteristics were similar between unclassifiable and type 2 diabetes, but the prevalence of microvascular complication in unclassifiable group had no significant difference compared with type 1 diabetes. The prevalence of macrovascular complications was significantly higher in the order of type 1, unclassifiable, and type 2 diabetes.