The paper reviews personal studies aimed at identifying clinical markers of eosinophil inflammation in allergic diseases. Preliminary data on the use of flow cytometry as a method to detect eosinophil activation through phenotypic activation markers are reported. The concept of eosinophil releasibility is introduced on the basis of in vitro eosinophil activation and mediator release by different stimuli. Data showing an increase of serum levels of ECP mainly in subjects with sensitivity to perennial allergens and related to histamine bronchial reactivity are discussed in view of their practical impact in diagnosing and monitoring of allergic diseases and asthma.