Mechanisms by which mesenchymal-derived tissue lineages participate in amplifying and perpetuating synovial inflammation in arthritis have been relatively underinvestigated and are therefore poorly understood. Elucidating these processes is likely to provide new insights into the pathogenesis of multiple diseases. Leukotriene B(4) (LTB(4)) is a potent proinflammatory lipid mediator that initiates and amplifies synovial inflammation in the K/BxN model of arthritis. We sought to elucidate mechanisms by which mesenchymal-derived fibroblast-like synoviocytes (FLSs) perpetuate synovial inflammation. We focused on the abilities of FLSs to contribute to LTB(4) synthesis and to respond to LTB(4) within the joint. Using a series of bone marrow chimeras generated from 5-lipoxygenase(-/-) and leukotriene A(4) (LTA(4)) hydrolase(-/-) mice, we demonstrate that FLSs generate sufficient levels of LTB(4) production through transcellular metabolism in K/BxN serum-induced arthritis to drive inflammatory arthritis. FLSs-which comprise the predominant lineage populating the synovial lining-are competent to metabolize exogenous LTA(4) into LTB(4) ex vivo. Stimulation of FLSs with TNF increased their capacity to generate LTB(4) 3-fold without inducing the expression of LTA(4) hydrolase protein. Moreover, LTB(4) (acting via LTB(4) receptor 1) was found to modulate the migratory and invasive activity of FLSs in vitro and also promote joint erosion by pannus tissue in vivo. Our results identify novel roles for FLSs and LTB(4) in joints, placing LTB(4) regulation of FLS biology at the center of a previously unrecognized amplification loop for synovial inflammation and tissue pathology.