With obesity, increased insulin secretion is needed to compensate for the additional demands and to maintain euglycemia. In contrast to Caucasians, the majority of type 2 diabetic patients belong to the non-obese category in Korea. There appears to be an ethnic difference underlying the pathogenesis in type 2 diabetes mellitus. However, there is only limited data on these subjects. The degree of insulin resistance in 267 Korean non-obese (body mass index [BMI] < 25 kg/m(2)) >/= patients with type 2 diabetes mellitus was analyzed, and the factors responsible for the insulin resistance were examined. The mean age and BMI of the patients were 50.8 +/- 10.6 years and 22.6 +/- 1.8 kg/m(2). Homeostasis model assessment-insulin resistance (HOMA-IR) >/= 2.5 was defined as being insulin resistant according to our data (mean +/- 1.5 SD of 1,917 normal subjects). There was no significant difference according to age, the duration of disease, and the glycosylated hemoglobin (HbA(lc)) levels between the subjects with or without insulin resistance. The HOMA-IR values in the patients with insulin resistance and normal insulin sensitivity were 4.2 +/- 1.4 and 1.5 +/- 0.6, respectively. In the insulin-resistant group, the log-transformed triglyceride (TG) levels were higher and the high-density lipoprotein-cholesterol (HDL-C) levels were lower than those of the insulin-sensitive group (log-transformed TG: 5.2 +/-.6 v 4.9 +/-.7 and HDL-C: 1.13 +/- 0.3 v 1.25 +/- 0.3mmol/L). These differences were still observed after adjusting for BMI. The HOMA-IR value was independently predicted by BMI and HDL-C levels, which explained 7% and 3% in the variability of insulin resistance, respectively. However, the TG levels were not independently associated with the HOMA-IR. Logistic regression analysis showed that the significant factor associated with HOMA-IR was only BMI. These results suggest that the BMI is the most important determinant of insulin resistance, while TG and HDL-C levels might be good markers of insulin resistance in non-obese patients with type 2 diabetes mellitus in Korea.