The purpose of this study was to assess the efficacy and safety of vildagliptin 50 mg bid add-on therapy to improve overall glycemic control in patients with T2DM inadequately controlled by insulin, with or without concomitant metformin treatment. It was agreed with PMDA to conduct a postmarketing clinical trial to further collect the efficacy and safety data of vildagliptin especially in Japanese patients when it iwas used on top of insulin.