The leukotrienes (LTs) are a novel group of biologically active mediators derived from arachidonic acid via lipoxygenase enzymes. LTB4 is a potent chemotactic agent for polymorphonuclear leukocytes and in vivo may mediate inflammatory reactions by inducing leukocyte recruitment by mediating indirectly vascular permeability charges and by modulating pain responses. LTC4 and LTD4 collectively account for the biological activity known as slow-reacting substance of anaphylaxis and are potent smooth muscle contracting agents. They may mediate inflammatory reactions by producing changes in blood flow and increases in vascular permeability. Evidence for LT involvement in a number of pathological conditions including diseases such as asthma, psoriasis, ulcerative colitis, and gout is now accumulating.