OBJECTIVE: To investigate the efficacy and safety of acarbose as add-on therapy in overweight type 2 patients with diabetes inadequately controlled by metformin. RESEARCH DESIGN AND METHODS: This study adopted a multicenter, randomized, double-blind, placebo-controlled, parallel group design. After a 4-week placebo run-in period, subjects were randomized to either acarbose (titrated up to 100 mg b.i.d.) or placebo. The primary efficacy variable was the change in HbA(1c) from baseline to the end of the 24-week treatment period. Change in fasting blood glucose was assessed as a secondary efficacy parameter. RESULTS: The intention-to-treat analysis from baseline to week 24 (81 patients for HbA(1c) and 82 for fasting blood glucose) showed statistically significant differences between acarbose and placebo treatment in HbA(1c) (1.02%; 95% CI 0.543-1.497; P = 0.0001) and fasting blood glucose (1.132 mmol/l; 95% CI 0.056-2.208; P = 0.0395) (adjusted least square means). In all, 18 patients (47%) in the acarbose group were classified as responders with a > or =5% reduction in HbA(1c) (relative to baseline) at the end point compared to 6 (14%) in the placebo group (P = 0.001). The safety profiles were similar for both treatment groups except for the higher incidence of gastrointestinal side effects during acarbose therapy. CONCLUSIONS: The addition of acarbose to metformin monotherapy provides an efficacious and safe alternative for glycemic improvement in overweight type 2 patients inadequately controlled by metformin alone.