Leukotrienes (LT), which are derived from arachidonic acid and have similar chemical structures to the prostaglandins, have recently been shown to contain most of the biological activity previously attributed to "Slow Reacting Substance of Anaphylaxis" (SRS-A). LTC4, LTD4 and LTE4 have potent bronchoconstrictor actions. LTE4 causes very long-lasting bronchoconstriction. LTB4 has inflammatory properties and possesses powerful chemotactic activity on leukocytes, especially neutrophil polymorphonuclear cells. The involvement of leukotrienes in the pathogenesis of bronchial asthma rests on a diversity of experimental evidence. The recent physiopathological approaches raise the hope of new therapeutic avenues in the treatment of asthma.