Chronic activation of the acute phase response (APR) is associated with atherosclerosis. Elevated levels of interleukin-6, the major inducer of the APR, are associated with an increased risk of cardiovascular events. One of the clinical hallmarks of atherogenesis is endothelial dysfunction, characterized by a decrease in endothelial production of nitric oxide (NO). We hypothesized that interleukin-6 (IL-6) decreases endothelial NO synthase (eNOS) expression. We now show that IL-6 treatment of human aortic endothelial cells (HAEC) decreases steady-state levels of human eNOS mRNA and protein. This decrease in eNOS expression is caused in part by IL-6 inhibition of transactivation of the human eNOS promoter. To explore the mechanism by which IL-6 affects eNOS expression, we examined activation of signal transducer and transactivator-3 (Stat3). The IL-6 receptor (IL-6R) is expressed in HAEC, and Stat3 is phosphorylated in response to IL-6 stimulation of the IL-6R. We identified four consensus sequences for Stat3 binding (SIE) in the eNOS promoter at positions -1520, -1024, -840, and -540. Transfection of eNOS promoter mutants revealed that the SIE at -1024 mediates Stat3 inhibition of eNOS promoter activity. Gel-shift analysis of nuclear extracts from HAEC treated with IL-6 confirms that Stat3 binds to a complex containing the SIE at -1024. RNA silencing of STAT3 blocks the inhibitory effect of IL-6 on eNOS expression. Our data show that IL-6 has direct effects upon endothelial cells, inhibiting eNOS expression in part through Stat3. Decreased levels of eNOS may be an important component of the pro-atherogenic effect of the APR.