This study evaluated the efficacy of rosiglitazone in non-obese and obese Korean type 2 diabetic patients of long duration. A total of 125 patients (M:F=44:81, mean age: 58.4+/-9.1 years, BMI: 24.2+/-2.7 kg/m2, duration of diabetes: 11.0+/-6.4 years) were randomly allocated to 12 weeks of rosiglitazone treatment (4 mg per day) or a control group. Responders were defined as patients who experienced fasting plasma glucose (FPG) reduction of >20% or HbA1c reduction of >1 (%). Rosiglitazone significantly improved glycemic control by reducing FPG and HbA1c (-3.4 mmol/l and -1.1%, P<0.001, respectively). It also significantly increased HOMA(beta-cell function) (+9.7, P<0.01) and QUICKI (+0.029, P<0.001), and decreased HOMA(IR) (-1.73, P<0.001). Females and those with higher waist-hip ratio made up a greater portion of rosiglitazone-responders. Responders (45 patients, 75%) also showed significantly higher FPG, HbA1c, systolic blood pressures, fasting insulin levels and HOMA(IR), and lower QUICKI than nonresponders. Among these parameters of responders, waist-hip ratio of non-obese subgroup, initial glycemic control of obese subgroup, and systolic blood pressure of both subgroups lost their significance after subdivision analysis. However, the baseline HOMA(IR) and QUICKI were significantly correlated with the response rate to rosiglitazone. Moreover, in multiple logistic regression analysis, HOMA(IR) and QUICKI retained their significance as the independent predictors. Even in Korean type 2 diabetic patients of long duration but with relatively preserved beta-cell function, rosiglitazone improved glycemic control, insulin sensitivity, and beta-cell function. In this ethnic group, female gender, central obesity, and especially severe insulin resistance were identified as predictive clinical parameters of rosiglitazone-responders.