The study was planned to determine the efficacy and safety of adding rosiglitazone to a combination of glimepiride and metformin therapy with insufficiently controlled type 2 diabetes. This was an open-label study with a follow-up period of 26 weeks. Thirty patients were taking 3 mg glimepiride two times and 850 mg metformin two times per day. Patients were told to take one rosiglitazone 4 mg tablet before breakfast additionally. The primary efficacy measure was the mean change in HbA₁c from baseline to the end of the study. Secondary efficacy parameters included the mean changes from baseline to the end of the study in fasting plasma glucose (FPG) and insulin levels, as well as total cholesterol, HDL-C, LDL-C, triglycerides, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels. Mean HbA₁c levels decreased significantly from 7.54 ± 0.9% to 6.57 ± 0.7% (p<0,001) at 26th week. FPG levels fell from 169.39 ± 37.8 mg/dl to 135.69 ± 28.0 mg/dl (p<0.001), respectively. Insulin levels decreased from 19.60 ± 9.8 U/L to 14.66 ± 11.6 U/L (p = 0,026) at 26th week. No one experienced elevations of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels greater than 2.5 times the upper limit of the reference range. This study confirms that the addition of rosiglitazone (4 mg/day) to sulphonylurea and metformin treatment for patients with type 2 diabetes improves glycemic control, is safe, and generally well tolerated.