The aim of this study was to evaluate the presence of PAF-acether (PAF), its specific degrading enzyme acetylhydrolase, and tumor necrosis factor (TNF) concentrations in blood and synovial fluid (SF) from patients with active RA. The variations of the mediators were also evaluated after corticosteroid perfusions in 7 patients. Lipo-PAF (PAF complexed to lipoproteins) was the main form of PAF detected both in blood and in SF, whereas unbound PAF was uncommon. Acetylhydrolase activity was also present in SF, with a strong correlation between serum and SF levels. TNF was detected in most of the samples, and TNF and acetylhydrolase levels were strongly correlated both in blood and in SF. Despite dramatic clinical improvement, corticosteroid treatment was not accompanied by a significant reduction of the concentration of blood mediators, suggesting that these molecules should not be considered as markers of disease activity.