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Objective: To evaluate the efficacy and safety of vildagliptin, a new dipeptidyl peptidase-4 inhibitor, added to metformin during 24-week treatment in patients with type 2 diabetes. Research Design and Methods: Double-blind, randomized, multicenter, parallel group study of 24-week treatment with vildagliptin 50 mg daily (n = 177), 100 mg daily (n = 185), or placebo (n = 182) in patients continuing a stable metformin dose regimen (≥1500 mg/day) but achieving inadequate glycemic control (HbA1c = 7.5%-11%). Results: The between-treatment difference (vildagliptin – placebo) in adjusted mean change (AM∆) in HbA1c from baseline to endpoint was-0.7 ± 0.1 % (P <.001) and-1.1 ± 0.1 % (P <.001) in patients receiving vildagliptin 50 mg or 100 mg daily, respectively. The between-treatment difference in the AM ∆ fasting glucose (FPG) was-0.8 ± 0.3 mmol/L (P =.003) and-1.7 ± 0.3 mmol/L (P <.001) in patients receiving vildagliptin 50 mg or 100 mg daily, respectively. Adverse events (AEs) were reported by 63.3%, 65.0%, and 63.5 % of patients receiving vildagliptin 50 mg daily, 100 mg daily, or placebo, respectively. Gastrointestinal AEs were reported by 9.6 % (P =.022 vs placebo), 14.8%, and 18.2 % of patients receiving vildagliptin 50 mg daily, 100 mg daily, or placebo