Sitagliptin is an oral, potent, highly selective, once-daily DPP-4 inhibitor indicated for the treatment of type 2 diabetes mellitus (T2DM). To assess the dose-ranging efficacy and safety/tolerability profile of once-daily sitagliptin 25, 50, 100, and 200 mg in Japanese patients with T2DM. In this randomized, double-blind, placebo-controlled study, 363 Japanese patients with inadequate glycemic control (HbA(1c)=6.5-10%; FPG< or =270 mg/dL) were randomized (1:1:1:1:1) to placebo, sitagliptin 25, 50, 100, or 200 mg q.d. for 12 weeks. The primary endpoint was change from baseline in HbA(1c) at Week 12. At Week 12, treatment with sitagliptin at all doses tested provided significant (p<0.001) reductions in HbA(1c) (-0.69 to -1.04%) from baseline (7.49 to 7.65%) relative to placebo. Sitagliptin significantly (p<0.001) reduced fasting plasma glucose (FPG; -15.9 to -23.2 mg/dL) and 2-hour postprandial glucose (2-hr PPG; -40.3 to -65.0 mg/dL) relative to placebo, in a dose-dependent manner. At doses > or =50 mg, differences in HbA(1c), FPG, and 2-hr PPG between the sitagliptin groups were not statistically significant. Sitagliptin was generally well tolerated with a low and similar incidence of hypoglycemia and minimal weight gain relative to placebo. Treatment with sitagliptin for 12 weeks provided significant and clinically meaningful reductions in HbA(1c), FPG, and 2-hr PPG across the dose range studied and was generally well tolerated in Japanese patients with T2DM.