Objectives: There is extensive but controversial evidence on the diverse effects of statins on the level of highdensity lipoprotein cholesterol (HDL-C). Some of these effects may limit the benefits of statins in terms of cardiovascular risk reduction. To identify the conditions for beneficial effects, this study investigated the response to atorvastatin and simvastatin treatment in type 2 diabetic patients with elevated low-density lipoprotein cholesterol (LDL-C). Methods: 2,872 subjects with type 2 diabetes from a disease management program were investigated. Patients with LDL-C ≥130 mg/dl or total cholesterol ≥200 mg/dl were put onto statin therapy by the National Health Insurance system in Taiwan. Results: 1,080 patients who completed 1 year of statin treatment were analyzed. There were significant reductions in LDL-C in both the atorvastatin (37.1%) and simvastatin (34.3%) group after one year of treatment compared with baseline levels. Unexpectedly, the majority of diabetic patients who received atorvastatin or simvastatin did not show an increase in HDL-C levels. 59.8% of patients had a significant HDL-C reduction (ΔHDL-C ≤ -3%) after atorvastatin treatment. Multivariate logistic regression analysis showed that the following patients were at higher risk of HDL-C reduction after 12 months: (i) patients in whom statin therapy was initiated aged <65 years and who had a BMI ≥24 kg/m2, (ii) male patients with a baseline HDL-C >40 mg/dl, and (iii) female patients with a baseline HDL-C >50 mg/dl. However, diabetic patients with severe atherogenic dyslipidemia (LDL-C ≥130, TG ≥204, and HDL-C ≤34 mg/dl) obtained more benefits in terms of HDL-C change after statin therapy. Conclusion: Diabetic patients, except those with severe atherogenic dyslipidemia, are prone to a decrease in serum HDL-C level after statin treatment, particularly after atorvastatin treatment.