BACKGROUND: Acarbose and voglibose are alpha-glucosidase inhibitors. Although different pharmacological effects and adverse abdominal events associated with the two drugs have been reported, no study directly compared acarbose and voglibose in diabetes has been undertaken. To compare the pharmacological effects and gastrointestinal adverse events between two drugs, a randomized, placebo-controlled, double-blind study was performed in type 2 diabetes patients. METHODS: The period of study was 12 weeks (observation period: 4 weeks; treatment period: 8 weeks). Fifty-three patients were randomized into two groups (the acarbose group: 24 patients; the voglibose group: 29 patients). The serum glucose, insulin, fructosamine, HbA1c, cholesterol, triglyceride and the incidence of adverse events were measured. RESULTS: 1) The reduction of glucose from before treatment to 4 weeks after treatment was significantly higher in the acarbose group, but the change before treatment and 8 weeks after treatment in the two groups was similar (p = 0.569). 2) The insulin significantly decreased after voglibose treatment (p = 0.040). 3) HbA1c level tended to decrease in voglibose group, and there was a significant decrease after acarbose treatment. However, the change in HbA1c level before and after treatment was similar between the two groups (p = 0.412). 4) The two drugs did not cause any other changes in the total, HDL-cholesterol and triglyceride. 5) The number of patients with gastrointestinal adverse events was significantly low 4 weeks after voglibose treatment (p = 0.049), but the incidence in the two groups was similar after 8 weeks (p = 0.215). CONCLUSIONS: Acarbose and voglibose significantly improved postprandial hyperglycemia in diabetes. The incidence of gastrointestinal adverse events was low 4 weeks after voglibose treatment.