We previously showed that endothelial cells (EC) from the vasculature of human solid tumors have a decreased expression of intercellular adhesion molecule-1 (ICAM-1) and ICAM-2 as compared with normal tissue EC. This effect is explained by EC exposure to angiogenic factors. It is known that upregulation of endothelial adhesion molecules (EAM) is a sign of EC activation in inflammatory responses. We therefore tested the effect of angiogenic factors on upregulation of EAM on tumor EC and human umbilical vein EC (HUVEC) by proinflammatory cytokines. Incubation of tumor-derived EC in tumor necrosis factor alpha (TNF alpha) did result in expression levels of only 20% of the level of similarly treated normal tissue-derived EC. Pretreatment of HUVEC with 10 ng/ml basic fibroblast growth factor (bFGF) for 3 days, before TNF alpha- or interleukin-1 alpha (IL-1 alpha) stimulation, resulted in ICAM-1 levels of only 30% to 60% of cells without pretreatment. Also, the induction of vascular EC adhesion molecule-1 (VCAM-1) and E- selectin by TNF alpha was significantly inhibited by prior exposure to bFGF. Vascular endothelial growth factor had similar but less prominent effects. The effect of transforming growth factor-beta and IL-8 was studied as well. The functional relevance of the finding of a decreased EC inflammatory response was confirmed by adhesion assays. Our results show that tumor angiogenesis induces EC anergy. This may serve as a tumor-protecting mechanism by impairing the development of an efficient leukocyte infiltrate in tumors.