Endothelial dysfunction is considered to be an early event in the development of atherosclerosis. The present study was undertaken to evaluate endothelial function and biochemical markers in type 2 diabetes mellitus (T2DM) patients before and after treatment with or without pioglitazone (PIO). Forty-one T2DM patients without macroangiopathy were randomized to treatment with (n=20) or without (control, n=21) PIO for 12 weeks. Endothelial function was assessed by flow-mediated vasodilation (FMD) using a high-resolution ultrasound method before and after treatment. After treatment, HbA1c levels equally decreased in both groups, but PIO-treated group had significantly increased high-density lipoprotein cholesterol (HDL-C) levels, and decreased triglyceride, fasting insulin levels and HOMA-R. After treatment, increases in %FMD, plasma HDL-C and adiponectin (APN) levels were significantly greater in PIO-treated group than those in control group. Changes of %FMD showed significant positive correlations with those of plasma APN and HDL-C levels. In conclusion, the present study showed that treatment of T2DM improved endothelial function with greater increases in %FMD, APN and HDL-C levels in PIO-treated group than those in control group, suggesting the beneficial effect of PIO on endothelial function in T2DM.