Osteoarthritis (OA) has been considered a biomechanically driven, degenerative disease of cartilage. However, the OA disease process affects not only the cartilage, but also the entire joint structure; and within the bone, cartilage and synovium of affected joints, profound metabolic changes transpire, which include the production of growth factors, nitric oxide (NO), prostaglandins (PGs), leukotrienes (LTs), IL-1β, tumor necrosis factor alpha, IL-6, and IL-8. The autocrine production of IL-1β by OA cartilage has been of particular interest, since both ex vivo human and in vivo animal studies indicate that IL-1 antagonists effectively attenuate cartilage degradation. Microarray technology has demonstrated differential expression in OA cartilage of a variety of IL-1-induced, NFβB-dependent genes. Among IL-β-induced products of OA cartilage are various eicosanoids, which include E₂, PGD₂, LTB₄, PGF_1α, PGF_2α and thromboxane. Treatment of OA cartilage with cyclooxygenase (COX) inhibitors increases LTB₄ production threefold to fivefold, indicating shunting of arachidonate from the COX to the 5-LO pathway. Functional analyses of individual eicosanoids reveals that PGD₂, in contrast to its derivative PGJ₂, stimulates catabolic processes, including NO and PG production. Lipoxin and 15-epi-lipoxin are also spontaneously released by OA cartilage, where they act to inhibit the spontaneous production of NO, PGE₂, IL-8 and IL-6. Consistent with the notion that OA is not simply a degenerative disease of cartilage, gene expression analysis of circulating peripheral blood mononuclear cells (PBMCs) shows upregulation of mRNA for IL-1β, COX-2, IL-6, and IL-8 in OA (but not normal) PBMCs. OA PBMCs produce threefold to fivefold more PGE₂ in response to stimulation with IL-1β than do normal cells. Thus, PBMCs, like chondrocytes and synovial cells, are activated in OA, and merit evaluation as sensors of inflammatory processes in the OA joint.