Among the inflammatory cells involved in the pathogenesis of asthma, eosinophils have been recognized as highly significant participants in the late-phase of the inflammatory response. Bronchial challenge with allergen inducing inflammation and exacerbation of asthma results in activation of eosinophils and release of specific eosinophil mediators, including eosinophil cationic protein (ECP). Recent studies have demonstrated that activation of eosinophils and increase of their release of ECP occur in patients naturally exposed to allergen, and in patients having inflammatory exacerbations of asthma, including development of bronchial hyperreactivity. ECP is elevated during exacerbation of extrinsic and intrinsic asthma in direct relationship to concomitant decrease in pulmonary function and increasing asthma symptoms. The elevated serum levels of ECP decline subsequent to effective therapy. Monitoring modulation of ECP levels may be useful in evaluating the treatment of asthmatic patients and as a marker for the efficacy of therapy. Several investigations have strongly suggested that serial determination of ECP in asthmatics may be especially useful as an inflammatory correlate to the mechanical abnormalities assessed by determination of pulmonary function in asthmatic patients.