Metabolic alterations such as insulin resistance are thought to underlie the endothelial dysfunction and low grade inflammation found in morbid obesity. Twenty-six morbidly obese patients, aged 39.0 +/- 10.0 (mean +/- sd), were evaluated before and 4.2 +/- 0.8 months after bariatric surgery. A marked increment in the insulin sensitivity index (S(I)) and the endothelium-dependent vasodilatory response in a dorsal hand vein was observed after weight loss following bariatric surgery. Circulating levels of E-selectin, P-selectin, plasminogen activator inhibitor-1, and von Willebrand factor, which were higher than those in the control group, decreased significantly after surgery. Plasma vascular cell adhesion molecule-1, angiotensin-converting enzyme, intercellular adhesion molecule-1, thrombomodulin, and plasma and intraplatelet cGMP levels did not change after weight loss. All inflammatory markers were higher in morbidly obese patients. After surgery, C- reactive protein and sialic acid diminished, whereas circulating levels of IL-6, TNF-alpha, and its soluble receptors did not. Positive correlations were found between changes in adiposity and S(I) and changes in C-reactive protein and between changes in sialic acid and changes in endothelial function. In conclusion, a marked improvement in S(I), endothelial function, and low grade inflammation was observed in the weight-losing, morbidly obese patients after bariatric surgery. S(I) and adiposity appear to play roles in obesity-related, low grade inflammation that contribute to the endothelial dysfunction observed in morbid obesity