We evaluated the effect of sitagliptin on glycemic control, endogenous insulin secretion, and beta cell function in Japanese patients with type 2 diabetes mellitus (T2DM) receiving a combination of oral antidiabetics and basal insulin analog glargine (basal-supported oral therapy [BOT]). Twenty-one patients showing inadequate glycemic control with BOT were given dipeptidylpeptidase-4 inhibitor (DPP-4I) sitagliptin at 50 mg/day for 12 weeks. Clinical markers of glycemic control, HbA1c, glycated albumin (GA), and 1,5-anhydroglucitol (1,5-AG), were measured before and 4 and 12 weeks after the start of sitagliptin. A 2-hour morning meal test was performed upon enrollment and at 12 weeks, and plasma glucose (PG), serum C-peptide, and plasma intact proinsulin (PI) were measured. HbA1c, GA, and 1,5-AG at 4 and 12 weeks were significantly improved over enrollment levels. The area under the PG concentration curve (AUC-PG) during the meal test at 12 weeks was significantly reduced (from 350 ± 17 mg ･ hr/dL before sitagliptin treatment to 338 ± 21 mg ･ hr/dL [mean ± SE], P < 0.05,); the AUC-C-peptide was unchanged (from 3.4 ± 0.4 ng ･ hr/mL to 3.6 ± 0.5 ng ･ hr/mL). However, both fasting and 2-hour PI/C-peptide ratios at 12 weeks were significantly decreased (from 13.3 ± 2.3 to 11.1 ± 2.0 [P < 0 .05] and from 9.5 ± 1.6 to 5.3 ± 0.9 [P < 0.01], respectively). Adding sitagliptin to BOT in Japanese T2DM patients appears to improve glycemic control without increasing endogenous insulin secretion and to reduce fasting and 2-hour postprandial PI/C-peptide ratios.