To determine the clinical characteristics in adolescent type 2 diabetes (young diabetes, YDM) in Taiwan, we enrolled 11 males who were diagnosed with YDM before 19 years of age into our study. Another 11 patients with adult type 2 diabetes (mature age diabetes, MADM) who were diagnosed after the age of 40 were enrolled as compare group. Subjects from both groups were being treated with oral hypoglycemic agents only at the time of enrollment, and none of the subjects had a history of diabetic ketoacidosis. Plasma lipid levels were measured from the fasting plasma sample. A homeostasis model assessment was used to estimate insulin sensitivity (HOMA-S) and beta-cell function (HOMA-B). Frequent-sampled intravenous glucose tolerance test was also performed to measure the insulin sensitivity (S(I)), glucose effectiveness (E(G)), and acute insulin response after glucose load (AIR). After adjusting for age and BMI, fasting plasma glucose, total cholesterol, HDL-cholesterol, triglycerides, and HOMA-B levels were similar between two groups. The fasting plasma insulin and HOMA-S were significantly higher in YDM. However, the S(I), E(G) and AIR in both groups were also not significantly different. In conclusion, the early onset of diabetes in YDM may be due to the early deterioration of the S(I), E(G) and AIR with similar severity compared with MADM. The role of E(G) might be more important than previously thought in these patients. Finally, the YDM might be a subtype of type 2 diabetes.