Eosinophils, a prominent feature of asthma, are found in increased numbers in the circulation and sputum, usually in relation to the severity of asthma. As a consequence of these clinical observations, investigators now speculate that the eosinophil has a central role in the pathogenesis of asthma. Recent evidence has begun to confirm these speculations. The allergic reaction of the airway to antigen and the development of the late asthmatic reaction have provided a clinical model to study asthma and the contribution of eosinophils to bronchial reactivity. In the late asthmatic reaction, airway eosinophilia occurs. Through a series of independent observations, the following eosinophil-related events have been noted with the development of late asthmatic reactions. With either laboratory or natural exposure to antigen, eosinophilic chemotactic factors are released. Although the sources of eosinophil chemotaxis are multicellular, this is an early step in the attraction of eosinophils to the airway. As this process is initiated, a series of events occurs to cause eosinophils to arrive in the airway and promote obstruction, injury, and bronchial hyperresponsiveness. These steps include eosinophil migration through the vascular endothelium, upregulation of eosinophils (characterized by a change in cell density), adhesion of eosinophils to airway epithelium, and release of eosinophil toxic products. This presentation will review some of the eosinophil-dependent factors that can cause asthma. Furthermore, the eosinophil may be a good target for future therapeutic interventions.