Background: This study determined the efficacy and safety of once-daily oral alogliptin in patients from mainland China, Taiwan, and Hong Kong with type 2 diabetes mellitus. Methods: In this Phase 3 multicenter double-blind placebo-controlled 16-week trial, 506 patients were randomized to receive once-daily alogliptin 25mg or placebo: 185 in the monotherapy group, 197 in the add-on to metformin group, and 124 in the add-on to pioglitazone group. The primary efficacy variable was the change from baseline (CFB) in HbA1c at Week 16; other efficacy measures included CFB to Week 16 in fasting plasma glucose (FPG), incidence of marked hyperglycemia (FPG 11.1mmol/L), and the incidence of clinical HbA1c 6.5% (48mmol/mol) and 7.0% (53mmol/mol) at Week 16. Safety was assessed throughout the trial. Results: Alogliptin monotherapy provided a significantly greater decrease in HbA1c from baseline to Week 16 compared with placebo (-0.58%; 95% confidence interval [CI] -0.78%, -0.37%; P < 0.001). As an add-on to metformin or pioglitazone, alogliptin also significantly decreased HbA1c compared with placebo (-0.69% [95% CI -0.87%, -0.51%; P < 0.001] and -0.52% [95% CI -0.75%, -0.28%; P < 0.001], respectively). In any treatment group versus placebo, alogliptin led to greater decreases in FPG (P0.004) and a higher percentage of patients who achieved an HbA1c target of <= 6.5% and <= 7.0% (P <= 0.003). No weight gain was observed in any treatment group. A similar percentage of patients experienced drug-related, treatment-emergent adverse events in the alogliptin and placebo arms. Four and two patients in the alogliptin and placebo arms, respectively, experienced mild or moderate hypoglycemia. Conclusions: Alogliptin 25mg once daily reduced HbA1c and FPG and enhanced clinical response compared with placebo when used as monotherapy or as an add-on to metformin or pioglitazone. Therapy with alogliptin was well tolerated.