Platelet activating factor (PAF) is a phospholipid with a strong inflammatory mediator. PAF is synthesized in a variety of cells in response to inflammatory stimuli. PAF is degraded by intracellular and extracellular PAF-acetylhydrolases (PAF-AHs) thus providing proper level of PAF. Plasma PAF-AH deficiency is associated with several diseases such as asthma, systemic lupus erythematosus, juvenile rheumatoid arthritis, acute myocardial infarction, diabetes, and membranous nephropathy. Cloning of plasma PAF-AH gene enabled the use of recombinant PAF-AH as a therapeutic tool for these PAF-mediated diseases.