For more than a century, it has been widely recognized that eosinophils are strongly associated with asthma. The degree of blood and Sputurn cosinophilia in asthmatic patients correlates with disease severity, airway responsiveness and response to glucocorticosteroids. Since eosinophils are prominent in allergic inflammation, and a single cytokine, interleukin-5 (IL-5), plays a central role in the maturation, release, and activation of eosinophils, its inhibition in man is a very attractive and novel approach to the treatment of allergic inflammation.