Among the lipid mediators, leukotrienes and the platelet activating factor (PAF) have been attributed various roles in the immunopathology of allergic diseases. PAF has been shown to be a chemotactic factor for eosinophils, which conversely release leukotriene C4 on activation. PAF also may induce airway hyperreactivity. Leukotrienes (LTB4) are also powerful chemotactic factors and predominantly attract neutrophils, which stimulates dependent release lipid mediators (hydroxyeicosatetraenoic acids--HETES, leukotrienes, PAF) and thus amplify the inflammatory process. Leukotrienes (LTC4, D4, E4) induce bronchoconstriction, and mucus production. Lipid mediators are released by immunological as well as nonimmunological processes, e.g. by inflammatory stimuli. While analytical methods for leukotriene determinations have been established, the analysis of PAF is mainly carried out by biological studies. Further investigations with antagonists may be helpful to clarify the role of the lipid mediators during allergic and inflammatory processes.