After years of research, the components of slow-reacting substance of anaphylaxis have been identified as the cysteinyl leukotrienes C4, D4, and E4. Leukotrienes are now known to be important mediators of chronic asthma. Leukotrienes cause bronchoconstriction, oedema, and mucus secretion in models of asthma and are produced in excess quantities in asthmatic patients. Leukotriene receptor antagonists and biosynthesis inhibitors have been produced to improve the signs and symptoms of asthma. These agents block laboratory challenges simulating chronic asthma such as exercise, allergen, and aspirin challenge. They also are effective in studies of chronic asthma in a wide variety of patient types demonstrating that leukotrienes are indeed important mediators of asthma. Over the next several years the appropriate place in asthma therapy for these new agents will be defined.