Atorvastatin is frequently administered for the treatment of hypercholesterolemia associated with type 2 diabetes mellitus. However, a marked deterioration of glycemic control has been reported in some patients treated with atorvastatin. No study has been done to determine whether atorvastatin adversely affects glycemic control. In this study, we retrospectively compared an atorvastatin-treated group (Group A, n = 76) with a pravastatin-treated group (Group P, n = 78) to examine the effects of the 2 statins on glycemic control from the onset of administration to 3 months thereafter. No change occurred in the antidiabetic drug dose in 62 patients of Group A and 68 patients of Group P. In those patients, arbitrary blood glucose levels increased from 147 ± 50 (mean ± SD) mg/dL to 177 ± 70 mg/dL in Group A and from 140 ± 38 mg/dL to 141 ± 32 mg/dL in Group P. HbA_1c increased from 6.8 ± 0.9% to 7.2 ± 1.1% in Group A and from 6.9 ± 0.9% to 6.9 ± 1.0% in Group P. The increase was significant only in Group A, and the extent of the increase was also significantly greater in Group A. These results suggest a predisposition to a deterioration of glycemic control in type 2 diabetic patients treated with atorvastatin.