Insulin resistance is a possible major metabolic cause of atherosclerosis. Endothelial dysfunction is commonly found in patients with insulin resistance, and primary treatment of insulin resistance with troglitazone should improve such endothelial dysfunction. Thus, the effects of troglitazone on endothelial function were investigated. Thirteen non-diabetic male subjects with hyperinsulinemic response to oral glucose load (n = 7) and normal (n = 6) subjects were investigated. Flow-mediated dilatation (FMD) of the brachial artery was examined by high resolution ultrasonography before and after the administration of troglitazone of 400 mg for 4 weeks. In insulin resistant subjects, fasting glucose (4.9+/-0.3 to 4.7+/-0.3 mmol/L, p<0.05), insulin (45+/-30 to 25+/-15 pmol/L, p<0.05) and response to oral glucose load (AUC glucose: 15.0+/-3.5 to 13.0+/-2.2 mmol x h/L, p<0.05; AUC insulin: 965+/-560 to 475+/-275 pmol x h/L, p<0.05) were significantly reduced. FMD was significantly improved in insulin resistant subjects. A significant negative correlation was observed between FMD and AUC insulin (r=-0.64, p<0.05). The present study demonstrates that FMD is impaired in insulin resistant subjects, and troglitazone improves the blunted vascular response and impaired insulin response. This finding suggests that primary treatment of insulin resistance could prevent the development of atherosclerosis by improving endothelial dysfunction.