Septic shock includes blood vessel dilatation and activation of innate immunity, which in turn causes release of antimicrobial peptides such as LL-37. It has been shown that LL-37 can attract leucocytes via the lipoxin A(4) receptor (ALX, FPRL1). ALX is also present in vascular endothelial cells. To explore possible ways of pharmacological intervention in septic shock, we investigated if LL-37 can affect vascular tone.Human omental arteries and veins were obtained during abdominal surgery, and circular smooth muscle activity was studied in organ baths. Gene expression was studied using reverse transcriptase-polymerase chain reaction.LL-37, at micromolar concentrations, induced a concentration- and endothelium-dependent relaxation in vein but not in artery segments precontracted by endothelin-1. The relaxation was profoundly reduced by potassium chloride (30 mM) to inhibit endothelium-derived hyperpolarizing factor (EDHF), whereas it was less affected by the NOS inhibitor, l-N(G)-nitroarginine methyl ester, and not at all by indomethacin. The ALX agonist, WKYMVm, also induced a relaxation and both the relaxations induced by LL-37 and WKYMVm were inhibited by the ALX antagonist, WRWWWW. ALX was expressed in the vein endothelium.We demonstrate, for the first time, that the human antimicrobial peptide, LL-37, induces endothelium-dependent relaxation in human omental veins mediated via an effect on endothelial ALX. The relaxation involves the release of nitric oxide and EDHF but not prostanoids. LL-37 released from white blood cells could contribute to blood vessel dilatation during sepsis and treatment with ALX antagonists might be successful.