Cysteinyl leukotriene has been known to be a major component of SRS-A (Slow Reacting Substance of Anaphylaxis). Inhibitors affecting on its biosynthesis and antagonists of its G-protein coupled receptor are therapeutic agents for acute inflammatory diseases, such as bronchial asthma and rhinitis. Latest findings in pathobiology of asthma suggest that cysteinyl leukotriene would take a key role in not only acute but also chronic inflammation of asthma. The structure basis of biosynthesis of cysteinyl leukotriene was revealed by crystallographic analysis of human membrane protein leukotriene C4 synthase.