Aberrant arachidonic acid (AA) metabolism has been involved in inflammation and carcinogenesis. The key enzymes in AA metabolism such as cytosolic phospholipase A2 (cPLA2) and cyclooxygenase-2 (COX-2) have been implicated in the development and progression of many human cancers, including lung cancer. Hence, the blockade of these enzymes may suppress promotion and survival of human cancer cells. We and others have shown that a natural triterpenoid, pachymic acid (PA), can exhibit antiinflammatory and anticancer properties; however, its potential mechanism has not been fully clarified. In this study, we examined the effect of PA on the proliferation of human nonsmall cell lung cancer A549 cells. Furthermore, we investigated the influences of nontoxic levels of PA on AA metabolism. Additionally, the cellular events and signal transduction pathways influenced by PA were also examined. Our results showed that PA (1) inhibited anchorage-dependent and -independent A549 growth in a concentration-dependent manner, (2) induced apoptosis and disrupted mitochondrial membrane potential in A549 cells, and at nonlethal levels, (3) decreased IL-1β-induced activation of cPLA2 and COX-2, (4) suppressed IL-1β-induced activation of mitogen-activated protein kinases (MAPKs), and (5) inhibited IL-1β-stimulated nuclear factor kappa B (NF-κB) signaling pathways. We speculate that inhibition of AA metabolism by PA is mediated in part by its inhibition of MAPKs and NF-κB signaling pathways. Our study reveals that, apart from its cytotoxic effect, PA has the chemopreventive potential by reducing production of eicosanoids from AA metabolism. © 2009 Wiley-Liss, Inc.