The aim of the study was to assess the efficacy/safety of once- (100 mg q.d.) or twice-daily (50 mg b.i.d.) sitagliptin 100 mg/day in Japanese patients with type 2 diabetes (T2DM). In this randomized, double-blind study, 80 patients with inadequate glycemic control (HbA1c=6.5-10%; FPG </=15.0 mmol/l) were randomized equally to sitagliptin 100 mg q.d., 50 mg b.i.d. or placebo for 4 weeks. At baseline and Week 4, frequent blood sampling was performed to assess 24-h weighted mean glucose (24-h WMG). Patients in the efficacy analyses (n=76) had a mean baseline HbA1c of 7.7%. At Week 4, least-squares mean changes in 24-h WMG were reduced with sitagliptin 100 mg q.d. and 50 mg b.i.d. versus placebo (-1.9, -1.6, and -0.5 mmol/l, respectively; p<0.001). Sitagliptin significantly improved FPG and 2-h PPG compared to placebo. No significant differences in 24-h WMG, FPG, or 2-h PPG were observed between the sitagliptin groups. Sitagliptin was well tolerated with no hypoglycemic events. In Japanese patients with T2DM, sitagliptin 100 mg/day provided substantial and continuous 24-h glucose-lowering over 4 weeks. The same glucose-lowering efficacy and tolerability were observed with sitagliptin 100 mg/day whether administered as a once-daily or twice-daily regimen. These results support a once-daily dosing regimen in Japanese patients with T2DM.