Combination therapy with a dipeptidyl peptidase (DPP)-4 inhibitor and metformin or sulfonylurea results in substantial and additive glucose-lowering effects in patients with type 2 diabetes mellitus (T2DM). However, it is not known whether triple combination therapy with a DPP-4 inhibitor, metformin, and sulfonylurea has greater additive effects or synergic effects. In the present report, we investigated the effect of addition of sitagliptin, the first-in-class DPP-4 inhibitor, to ongoing metformin and sulfonylurea therapy in three female Japanese patients with T2DM who refused insulin therapy. Combined treatment with all three drugs resulted in marked improvements in HbA1c. In the first patient, HbA1c levels decreased from 11.1% to 6.1% after the addition of sitagliptin to metformin 1000 mg, glibenclamide, and miglitol, even though the dose of glibenclamide was decreased. HbA1c levels decreased similarly in the second patient, who was being treated with metformin and glibenclamide, from 7.9% to 6.0% after addition of sitagliptin and an increase in metformin to 2250 mg; this patient ceased glibenclamide because of hypoglycemia and instead was started on low-dose glimepiride. In the third patient, HbA1c levels decreased from 8.6% to 7.1% after addition of glimepiride to ongoing sitagliptin and metformin therapy. All three patients had refused insulin therapy, despite the fact that ongoing combination therapy had failed to achieve satisfactory glycemic control. Based on these results, it is likely that the addition of sitagliptin to metformin and at least a small dose of sulfonylurea may be effective in reducing HbA1c levels without weight gain. This triple combination therapy may prove useful in at least some patients who need initiation of insulin therapy.