In this report, we demonstrate that NADPH oxidase is activated by tumor necrosis factor-a (TNF-a) plus interferon-g (IFN-g) in human monocytic cells (THP-1 cells) differentiated with phorbol ester (PMA) and that physiological concentration of 17h-estradiol inhibits NADPH oxidase activity in THP-1 cells stimulated with TNF-a plus IFN-g. This effect is mediated by estrogen receptor based on estrogen receptor antagonist (ICI 182, 780) that diminishes inhibition by 17h-estradiol. This inhibition is specific in 17h-estradiol because 17a-estradiol, testosterone and progesterone do not inhibit NADPH oxidase activity. Activation of NADPH oxidase induced by TNF-a plus IFN-g is caused by up-regulation of p47 (cytosolic component of NADPH oxidase) expression. 17h-Estradiol prevents the up-regulation of p47 mRNA and protein expression. This prevention of p47 expression depends on the inhibition of NF-nB activation. Our results implicate that 17h-estradiol has an anti-atherosclerotic effects through the improvement of nitric oxide (NO) bioavailability caused by the regulation of superoxide (O2 ) production. D 2003 Elsevier Science B.V. All rights reserved.