Interleukin 1 (IL-1) induces prostanoid biosynthesis in endothelial cells by promoting cyclooxygenase expression, but little is known about its activity on the biosynthesis of hydroxyeicosatetraenoic acids (HETEs). We studied the effect of human recombinant IL-1β on the conversion of arachidonic acid (AA) to 15-HETE, a powerful inhibitor of the biosynthesis of proinflammatory eicosanoids. Cultured human umbilical vein endothelial cells were incubated with or without IL-1β prior to the addition of labeled AA. The eicosanoids produced were analyzed by RP-HPLC. Untreated cells produced little amounts of 15-HETE (6 ± 3 pmol/10 6 cells), but IL-1β treated cells increased 15-HETE formation in a dose-dependent manner (4–5-fold at 10 U/ml IL-1). The production of HETEs by IL-1β was dependent on protein synthesis. Aspirin inhibited prostanoids, HHT and 11-HETE dose dependently, whereas it was unable to totally inhibit 15-HETE in IL-1β-treated cells (50−60%). Nordihydroguaiaretic acid, a general lipoxygenase inhibitor, preferably inhibited 15-HETE formation but also reduced the synthesis of the other eicosanoids in a dose-dependent manner. Indomethacin and ETYA completely suppressed prostanoids, 11-HETE and 15-HETE formation in resting and IL-1β-activated cells. Using specific 15-lipoxygenase oligonucleotides and the reverse transcriptase polymerase chain reaction technique, we were unable to evidence detectable 15-lipoxygenase mRNA both in resting and IL-1-activated endothelial cells. Overall, these results provide evidence that in human endothelial cells IL-1β increases 15-HETE production. Data strongly suggest that this effect is mediated by cyclooxygenase rather than 15-lipoxygenase activity or expression.