Objective: To assess the efficacy and tolerability of vildagliptin (10, 25 or 50 mg bid) in Japanese patients with type 2 diabetes mellitus (T2DM). Methods: This 12-week, multicenter, randomized, double-blind, placebo-controlled, parallel-group study was performed in 291 patients. The primary assessment was change from baseline to endpoint in HbA1c. Results: Baseline HbA1c averaged 7.4%, and the between-treatment difference (vildagliptin-placebo) in the HbA1c adjusted mean change was -0.8%, -1.0% and -1.2% with vildagliptin 10, 25 and 50 mg bid, respectively (p < 0.001). Relative to baseline, body weight did not change significantly in vildagliptin groups. There was no increase in incidence of adverse events in the vildagliptin groups (62.0%, 62.5% and 61.8%, 10, 25 and 50 mg bid, respectively) compared to placebo (73.6%). No deaths or drug-related serious adverse events were reported. Seven hypoglycemic events were observed (four events (n = 3), two events (n = 2), and one event (n = 1) in the vildagliptin 10 and 50 mg bid, and placebo, respectively) and none of them were severe or dose related. Conclusion: Vildagliptin 50 mg bid was considered to be the most effective and well-tolerated dose, and therefore can be considered the recommended clinical dose for Japanese patients with T2DM. (C) 2008 Elsevier Ireland Ltd. All rights reserved.