The general aim of the present study is to investigate the impact of 4 weeks treatment with linagliptin (5 mg) on endothelial function in patients with type 2 diabetes mellitus. In the current trial this effect of linagliptin treatment on endothelial function will be compared against both the sulfonylurea glimepiride and against placebo, which has not been tested in a trial before; as also is the case for other DPP-4 inhibitors.