Endothelial dysfunction is an early marker of atherosclerosis occurring in patients with type 2 diabetes mellitus. Endothelium-dependent dilation (EDD) has been shown to improve by combined therapy of insulin and metformin. Studies on endothelium-independent vasodilatory capacity, however, have had controversial results. We sought to investigate the vascular reactivity--EDD and endothelium-independent dilation--and their changes induced by the addition of insulin therapy to patients with type 2 diabetes mellitus pretreated with diet and oral hypoglycemic drugs. We therefore performed vascular studies in 21 poorly controlled type 2 diabetic patients and 11 nondiabetic control subjects by using high resolution ultrasound of the brachial artery. After 3 months of additional insulin therapy, vascular and laboratory measurements including C-reactive protein and parameters of glucose and lipoprotein metabolism were repeated. At baseline, EDD was significantly impaired in diabetic patients compared with controls (2.7 +/- 2.2% vs 7.0 +/- 1.8%, p <0.001), whereas endothelium-independent dilation was normal in both groups. After insulin therapy, EDD increased from 2.7 +/- 2.2% to 5.0 +/- 2.8% (p <0.001) in diabetic patients. All other vascular parameters did not change over the treatment period. The absolute change in EDD showed a significant negative correlation with the change in hemoglobin A(1c) (r = -0.67, p <0.001) and with fasting blood glucose (r = -0.84, p <0.001) levels. In contrast, there was no correlation between EDD and the observed changes in lipid and C-reactive protein levels. Our findings demonstrate that insulin therapy has beneficial effects on vascular function, resulting in enhanced EDD, most probably due to an improved glycemic control as the underlying mechanism.