Paeonol (2'-hydroxy-4'-methoxyacetophenone), the main active compound of the traditionally used Chinese herb Paeonia lactiflora Pallas, has anti-inflammatory, antioxidant and cardiovascular protective activities. We studied how the levels of intercellular adhesion molecule-1 (ICAM-1), one of the key molecules in the development of atherosclerosis, might be affected by paeonol in tumor necrosis factor-alpha (TNF-alpha)-activated human umbilical vein endothelial cells (HUVECs). Paeonol concentration-dependently inhibited the production of ICAM-1; it inhibited nuclear factor-kappaB (NF-kappaB) p65 translocation into the nucleus and the phosphorylation of inhibitory factor kappaBalpha (IkappaBalpha). It also blocked the TNF-alpha-induced phosphorylation of p38 and extracellular signal-regulated kinase (ERK), which are involved in regulating ICAM-1 production by TNF-alpha. Paeonol inhibited U937 monocyte adhesion to HUVECs stimulated by TNF-alpha, suggesting that it may inhibit the binding of monocytes to endothelium by regulating the production of critical adhesion molecules by TNF-alpha. The inhibitory effect of paeonol on ICAM-1 production might be mediated by inhibiting p38, ERK and NF-kappaB signaling pathways, which are involved in TNF-alpha-induced ICAM-1 production. Thus, paeonol may be beneficial in the treatment of cardiovascular disorders such as atherosclerosis.